--- a/testing/marionette/capabilities.js
+++ b/testing/marionette/capabilities.js
@@ -18,16 +18,17 @@ const {
 
 XPCOMUtils.defineLazyGlobalGetters(this, ["URL"]);
 
 this.EXPORTED_SYMBOLS = [
   "Capabilities",
   "PageLoadStrategy",
   "Proxy",
   "Timeouts",
+  "UnhandledPromptBehavior",
 ];
 
 // Enable testing this module, as Services.appinfo.* is not available
 // in xpcshell tests.
 const appinfo = {name: "<missing>", version: "<missing>"};
 try { appinfo.name = Services.appinfo.name.toLowerCase(); } catch (e) {}
 try { appinfo.version = Services.appinfo.version; } catch (e) {}
 
@@ -352,30 +353,55 @@ class Proxy {
       socksVersion: this.socksVersion,
       proxyAutoconfigUrl: this.proxyAutoconfigUrl,
     });
   }
 
   toString() { return "[object Proxy]"; }
 }
 
+/**
+ * Enum of unhandled prompt behavior.
+ *
+ * @enum
+ */
+const UnhandledPromptBehavior = {
+  /** All simple dialogs encountered should be accepted. */
+  Accept: "accept",
+  /**
+   * All simple dialogs encountered should be accepted, and an error
+   * returned that the dialog was handled.
+   */
+  AcceptAndNotify: "accept and notify",
+  /** All simple dialogs encountered should be dismissed. */
+  Dismiss: "dismiss",
+  /**
+   * All simple dialogs encountered should be dismissed, and an error
+   * returned that the dialog was handled.
+   */
+  DismissAndNotify: "dismiss and notify",
+  /** All simple dialogs encountered should be left to the user to handle. */
+  Ignore: "ignore",
+};
+
 /** WebDriver session capabilities representation. */
 class Capabilities extends Map {
   /** @class */
   constructor() {
     super([
       // webdriver
       ["browserName", appinfo.name],
       ["browserVersion", appinfo.version],
       ["platformName", getWebDriverPlatformName()],
       ["platformVersion", Services.sysinfo.getProperty("version")],
+      ["acceptInsecureCerts", false],
       ["pageLoadStrategy", PageLoadStrategy.Normal],
-      ["acceptInsecureCerts", false],
+      ["proxy", new Proxy()],
       ["timeouts", new Timeouts()],
-      ["proxy", new Proxy()],
+      ["unhandledPromptBehavior", UnhandledPromptBehavior.DismissAndNotify],
 
       // features
       ["rotatable", appinfo.name == "B2G"],
 
       // proprietary
       ["moz:accessibilityChecks", false],
       ["moz:headless", Cc["@mozilla.org/gfx/info;1"].getService(Ci.nsIGfxInfo).isHeadless],
       ["moz:processID", Services.appinfo.processID],
@@ -465,16 +491,29 @@ class Capabilities extends Map {
           matched.set("proxy", proxy);
           break;
 
         case "timeouts":
           let timeouts = Timeouts.fromJSON(v);
           matched.set("timeouts", timeouts);
           break;
 
+        case "unhandledPromptBehavior":
+          assert.string(v,
+              pprint`Expected ${k} to be a string, got ${v}`);
+
+          if (Object.values(UnhandledPromptBehavior).includes(v)) {
+            matched.set("unhandledPromptBehavior", v);
+          } else {
+            throw new InvalidArgumentError(
+                `Unknown unhandled prompt behavior: ${v}`);
+          }
+
+          break;
+
         case "moz:accessibilityChecks":
           assert.boolean(v,
               pprint`Expected ${k} to be a boolean, got ${v}`);
           matched.set("moz:accessibilityChecks", v);
           break;
 
         case "moz:useNonSpecCompliantPointerOrigin":
           assert.boolean(v,
@@ -493,16 +532,17 @@ class Capabilities extends Map {
     return matched;
   }
 }
 
 this.Capabilities = Capabilities;
 this.PageLoadStrategy = PageLoadStrategy;
 this.Proxy = Proxy;
 this.Timeouts = Timeouts;
+this.UnhandledPromptBehavior = UnhandledPromptBehavior;
 
 function getWebDriverPlatformName() {
   let name = Services.sysinfo.getProperty("name");
 
   switch (name) {
     case "Windows_NT":
       return "windows";
 

--- a/testing/marionette/driver.js
+++ b/testing/marionette/driver.js
@@ -15,16 +15,17 @@ ChromeUtils.import("chrome://marionette/
 const {
   browser,
   Context,
   WindowState,
 } = ChromeUtils.import("chrome://marionette/content/browser.js", {});
 const {
   Capabilities,
   Timeouts,
+  UnhandledPromptBehavior,
 } = ChromeUtils.import("chrome://marionette/content/capabilities.js", {});
 ChromeUtils.import("chrome://marionette/content/capture.js");
 const {
   CertificateOverrideManager,
   InsecureSweepingOverride,
 } = ChromeUtils.import("chrome://marionette/content/cert.js", {});
 ChromeUtils.import("chrome://marionette/content/cookie.js");
 const {
@@ -36,16 +37,17 @@ const {
   InsecureCertificateError,
   InvalidArgumentError,
   InvalidCookieDomainError,
   InvalidSelectorError,
   NoSuchAlertError,
   NoSuchFrameError,
   NoSuchWindowError,
   SessionNotCreatedError,
+  UnexpectedAlertOpenError,
   UnknownError,
   UnsupportedOperationError,
   WebDriverError,
 } = ChromeUtils.import("chrome://marionette/content/error.js", {});
 ChromeUtils.import("chrome://marionette/content/evaluate.js");
 const {pprint} = ChromeUtils.import("chrome://marionette/content/format.js", {});
 ChromeUtils.import("chrome://marionette/content/interaction.js");
 ChromeUtils.import("chrome://marionette/content/l10n.js");
@@ -1063,17 +1065,17 @@ GeckoDriver.prototype.execute_ = async f
  * @throws {NoSuchWindowError}
  *     Top-level browsing context has been discarded.
  * @throws {UnexpectedAlertOpenError}
  *     A modal dialog is open, blocking this operation.
  */
 GeckoDriver.prototype.get = async function(cmd) {
   assert.content(this.context);
   assert.open(this.getCurrentWindow());
-  this._assertAndDismissModal();
+  this._handleUserPrompts();
 
   let url = cmd.parameters.url;
 
   let get = this.listener.get({url, pageTimeout: this.timeouts.pageLoad});
 
   // If a reload of the frame script interrupts our page load, this will
   // never return. We need to re-issue this request to correctly poll for
   // readyState and send errors.
@@ -1105,17 +1107,17 @@ GeckoDriver.prototype.get = async functi
  *
  * @throws {NoSuchWindowError}
  *     Top-level browsing context has been discarded.
  * @throws {UnexpectedAlertOpenError}
  *     A modal dialog is open, blocking this operation.
  */
 GeckoDriver.prototype.getCurrentUrl = function() {
   assert.open(this.getCurrentWindow());
-  this._assertAndDismissModal();
+  this._handleUserPrompts();
 
   return this.currentURL.toString();
 };
 
 /**
  * Gets the current title of the window.
  *
  * @return {string}
@@ -1123,17 +1125,17 @@ GeckoDriver.prototype.getCurrentUrl = fu
  *
  * @throws {NoSuchWindowError}
  *     Top-level browsing context has been discarded.
  * @throws {UnexpectedAlertOpenError}
  *     A modal dialog is open, blocking this operation.
  */
 GeckoDriver.prototype.getTitle = function() {
   assert.open(this.getCurrentWindow());
-  this._assertAndDismissModal();
+  this._handleUserPrompts();
 
   return this.title;
 };
 
 /** Gets the current type of the window. */
 GeckoDriver.prototype.getWindowType = function() {
   assert.open(this.getCurrentWindow());
 
@@ -1149,17 +1151,17 @@ GeckoDriver.prototype.getWindowType = fu
  *
  * @throws {NoSuchWindowError}
  *     Top-level browsing context has been discarded.
  * @throws {UnexpectedAlertOpenError}
  *     A modal dialog is open, blocking this operation.
  */
 GeckoDriver.prototype.getPageSource = async function() {
   const win = assert.open(this.getCurrentWindow());
-  this._assertAndDismissModal();
+  this._handleUserPrompts();
 
   switch (this.context) {
     case Context.Chrome:
       let s = new win.XMLSerializer();
       return s.serializeToString(win.document);
 
     case Context.Content:
       return this.listener.getPageSource();
@@ -1178,17 +1180,17 @@ GeckoDriver.prototype.getPageSource = as
  * @throws {NoSuchWindowError}
  *     Top-level browsing context has been discarded.
  * @throws {UnexpectedAlertOpenError}
  *     A modal dialog is open, blocking this operation.
  */
 GeckoDriver.prototype.goBack = async function() {
   assert.content(this.context);
   assert.open(this.curBrowser);
-  this._assertAndDismissModal();
+  this._handleUserPrompts();
 
   // If there is no history, just return
   if (!this.curBrowser.contentBrowser.webNavigation.canGoBack) {
     return;
   }
 
   let lastURL = this.currentURL;
   let goBack = this.listener.goBack({pageTimeout: this.timeouts.pageLoad});
@@ -1220,17 +1222,17 @@ GeckoDriver.prototype.goBack = async fun
  * @throws {NoSuchWindowError}
  *     Top-level browsing context has been discarded.
  * @throws {UnexpectedAlertOpenError}
  *     A modal dialog is open, blocking this operation.
  */
 GeckoDriver.prototype.goForward = async function() {
   assert.content(this.context);
   assert.open(this.curBrowser);
-  this._assertAndDismissModal();
+  this._handleUserPrompts();
 
   // If there is no history, just return
   if (!this.curBrowser.contentBrowser.webNavigation.canGoForward) {
     return;
   }
 
   let lastURL = this.currentURL;
   let goForward = this.listener.goForward(
@@ -1263,17 +1265,17 @@ GeckoDriver.prototype.goForward = async 
  * @throws {NoSuchWindowError}
  *     Top-level browsing context has been discarded.
  * @throws {UnexpectedAlertOpenError}
  *     A modal dialog is open, blocking this operation.
  */
 GeckoDriver.prototype.refresh = async function() {
   assert.content(this.context);
   assert.open(this.getCurrentWindow());
-  this._assertAndDismissModal();
+  this._handleUserPrompts();
 
   let refresh = this.listener.refresh(
       {pageTimeout: this.timeouts.pageLoad});
 
   // If a reload of the frame script interrupts our page load, this will
   // never return. We need to re-issue this request to correctly poll for
   // readyState and send errors.
   this.curBrowser.pendingCommands.push(() => {
@@ -1406,17 +1408,17 @@ GeckoDriver.prototype.getChromeWindowHan
  *
  * @throws {NoSuchWindowError}
  *     Top-level browsing context has been discarded.
  * @throws {UnexpectedAlertOpenError}
  *     A modal dialog is open, blocking this operation.
  */
 GeckoDriver.prototype.getWindowRect = function() {
   assert.open(this.getCurrentWindow());
-  this._assertAndDismissModal();
+  this._handleUserPrompts();
 
   return this.curBrowser.rect;
 };
 
 /**
  * Set the window position and size of the browser on the operating
  * system window manager.
  *
@@ -1444,17 +1446,17 @@ GeckoDriver.prototype.getWindowRect = fu
  * @throws {NoSuchWindowError}
  *     Top-level browsing context has been discarded.
  * @throws {UnexpectedAlertOpenError}
  *     A modal dialog is open, blocking this operation.
  */
 GeckoDriver.prototype.setWindowRect = async function(cmd) {
   assert.firefox();
   const win = assert.open(this.getCurrentWindow());
-  this._assertAndDismissModal();
+  this._handleUserPrompts();
 
   let {x, y, width, height} = cmd.parameters;
   let origRect = this.curBrowser.rect;
 
   // Synchronous resize to |width| and |height| dimensions.
   async function resizeWindow(width, height) {
     return new Promise(resolve => {
       win.addEventListener("resize", whenIdle(win, resolve), {once: true});
@@ -1672,17 +1674,17 @@ GeckoDriver.prototype.getActiveFrame = f
  *
  * @throws {NoSuchWindowError}
  *     Top-level browsing context has been discarded.
  * @throws {UnexpectedAlertOpenError}
  *     A modal dialog is open, blocking this operation.
  */
 GeckoDriver.prototype.switchToParentFrame = async function() {
   assert.open(this.getCurrentWindow());
-  this._assertAndDismissModal();
+  this._handleUserPrompts();
 
   await this.listener.switchToParentFrame();
 };
 
 /**
  * Switch to a given frame within the current window.
  *
  * @param {Object} element
@@ -1693,17 +1695,17 @@ GeckoDriver.prototype.switchToParentFram
  *
  * @throws {NoSuchWindowError}
  *     Top-level browsing context has been discarded.
  * @throws {UnexpectedAlertOpenError}
  *     A modal dialog is open, blocking this operation.
  */
 GeckoDriver.prototype.switchToFrame = async function(cmd) {
   assert.open(this.getCurrentWindow());
-  this._assertAndDismissModal();
+  this._handleUserPrompts();
 
   let {id, focus} = cmd.parameters;
 
   // TODO(ato): element can be either string (deprecated) or a web
   // element JSON Object.  Can be removed with Firefox 60.
   let byFrame;
   if (typeof cmd.parameters.element == "string") {
     byFrame = WebElement.fromUUID(cmd.parameters.element, Context.Chrome);
@@ -1908,17 +1910,17 @@ GeckoDriver.prototype.singleTap = async 
  *     Top-level browsing context has been discarded.
  * @throws {UnexpectedAlertOpenError}
  *     A modal dialog is open, blocking this operation.
  */
 GeckoDriver.prototype.performActions = async function(cmd) {
   assert.content(this.context,
       "Command 'performActions' is not yet available in chrome context");
   assert.open(this.getCurrentWindow());
-  this._assertAndDismissModal();
+  this._handleUserPrompts();
 
   let actions = cmd.parameters.actions;
   await this.listener.performActions({"actions": actions});
 };
 
 /**
  * Release all the keys and pointer buttons that are currently depressed.
  *
@@ -1927,17 +1929,17 @@ GeckoDriver.prototype.performActions = a
  * @throws {NoSuchWindowError}
  *     Top-level browsing context has been discarded.
  * @throws {UnexpectedAlertOpenError}
  *     A modal dialog is open, blocking this operation.
  */
 GeckoDriver.prototype.releaseActions = async function() {
   assert.content(this.context);
   assert.open(this.getCurrentWindow());
-  this._assertAndDismissModal();
+  this._handleUserPrompts();
 
   await this.listener.releaseActions();
 };
 
 /**
  * An action chain.
  *
  * @param {Object} value
@@ -1951,17 +1953,17 @@ GeckoDriver.prototype.releaseActions = a
  *     Not applicable to application.
  * @throws {NoSuchWindowError}
  *     Top-level browsing context has been discarded.
  * @throws {UnexpectedAlertOpenError}
  *     A modal dialog is open, blocking this operation.
  */
 GeckoDriver.prototype.actionChain = async function(cmd) {
   const win = assert.open(this.getCurrentWindow());
-  this._assertAndDismissModal();
+  this._handleUserPrompts();
 
   let {chain, nextId} = cmd.parameters;
 
   switch (this.context) {
     case Context.Chrome:
       // be conservative until this has a use case and is established
       // to work as expected in Fennec
       assert.firefox();
@@ -1990,17 +1992,17 @@ GeckoDriver.prototype.actionChain = asyn
  * @throws {NoSuchWindowError}
  *     Top-level browsing context has been discarded.
  * @throws {UnexpectedAlertOpenError}
  *     A modal dialog is open, blocking this operation.
  */
 GeckoDriver.prototype.multiAction = async function(cmd) {
   assert.content(this.context);
   assert.open(this.getCurrentWindow());
-  this._assertAndDismissModal();
+  this._handleUserPrompts();
 
   let {value, max_length} = cmd.parameters; // eslint-disable-line camelcase
   await this.listener.multiAction(value, max_length);
 };
 
 /**
  * Find an element using the indicated search strategy.
  *
@@ -2011,17 +2013,17 @@ GeckoDriver.prototype.multiAction = asyn
  *
  * @throws {NoSuchWindowError}
  *     Top-level browsing context has been discarded.
  * @throws {UnexpectedAlertOpenError}
  *     A modal dialog is open, blocking this operation.
  */
 GeckoDriver.prototype.findElement = async function(cmd) {
   const win = assert.open(this.getCurrentWindow());
-  this._assertAndDismissModal();
+  this._handleUserPrompts();
 
   let {using, value} = cmd.parameters;
   let startNode;
   if (typeof cmd.parameters.element != "undefined") {
     startNode = WebElement.fromUUID(cmd.parameters.element, this.context);
   }
 
   let opts = {
@@ -2110,17 +2112,17 @@ GeckoDriver.prototype.findElements = asy
  *     A modal dialog is open, blocking this operation.
  * @throws {NoSuchElementError}
  *     If the document does not have an active element, i.e. if
  *     its document element has been deleted.
  */
 GeckoDriver.prototype.getActiveElement = async function() {
   assert.content(this.context);
   assert.open(this.getCurrentWindow());
-  this._assertAndDismissModal();
+  this._handleUserPrompts();
 
   return this.listener.getActiveElement();
 };
 
 /**
  * Send click event to element.
  *
  * @param {string} id
@@ -2132,17 +2134,17 @@ GeckoDriver.prototype.getActiveElement =
  *     If element represented by reference <var>id</var> is unknown.
  * @throws {NoSuchWindowError}
  *     Top-level browsing context has been discarded.
  * @throws {UnexpectedAlertOpenError}
  *     A modal dialog is open, blocking this operation.
  */
 GeckoDriver.prototype.clickElement = async function(cmd) {
   assert.open(this.getCurrentWindow());
-  this._assertAndDismissModal();
+  this._handleUserPrompts();
 
   let id = assert.string(cmd.parameters.id);
   let webEl = WebElement.fromUUID(id, this.context);
 
   switch (this.context) {
     case Context.Chrome:
       let el = this.curBrowser.seenEls.get(webEl);
       await interaction.clickElement(el, this.a11yChecks);
@@ -2191,17 +2193,17 @@ GeckoDriver.prototype.clickElement = asy
  *     If element represented by reference <var>id</var> is unknown.
  * @throws {NoSuchWindowError}
  *     Top-level browsing context has been discarded.
  * @throws {UnexpectedAlertOpenError}
  *     A modal dialog is open, blocking this operation.
  */
 GeckoDriver.prototype.getElementAttribute = async function(cmd) {
   assert.open(this.getCurrentWindow());
-  this._assertAndDismissModal();
+  this._handleUserPrompts();
 
   let id = assert.string(cmd.parameters.id);
   let name = assert.string(cmd.parameters.name);
   let webEl = WebElement.fromUUID(id, this.context);
 
   switch (this.context) {
     case Context.Chrome:
       let el = this.curBrowser.seenEls.get(webEl);
@@ -2232,17 +2234,17 @@ GeckoDriver.prototype.getElementAttribut
  *     If element represented by reference <var>id</var> is unknown.
  * @throws {NoSuchWindowError}
  *     Top-level browsing context has been discarded.
  * @throws {UnexpectedAlertOpenError}
  *     A modal dialog is open, blocking this operation.
  */
 GeckoDriver.prototype.getElementProperty = async function(cmd) {
   assert.open(this.getCurrentWindow());
-  this._assertAndDismissModal();
+  this._handleUserPrompts();
 
   let id = assert.string(cmd.parameters.id);
   let name = assert.string(cmd.parameters.name);
   let webEl = WebElement.fromUUID(id, this.context);
 
   switch (this.context) {
     case Context.Chrome:
       let el = this.curBrowser.seenEls.get(webEl);
@@ -2272,17 +2274,17 @@ GeckoDriver.prototype.getElementProperty
  *     If element represented by reference <var>id</var> is unknown.
  * @throws {NoSuchWindowError}
  *     Top-level browsing context has been discarded.
  * @throws {UnexpectedAlertOpenError}
  *     A modal dialog is open, blocking this operation.
  */
 GeckoDriver.prototype.getElementText = async function(cmd) {
   assert.open(this.getCurrentWindow());
-  this._assertAndDismissModal();
+  this._handleUserPrompts();
 
   let id = assert.string(cmd.parameters.id);
   let webEl = WebElement.fromUUID(id, this.context);
 
   switch (this.context) {
     case Context.Chrome:
       // for chrome, we look at text nodes, and any node with a "label" field
       let el = this.curBrowser.seenEls.get(webEl);
@@ -2313,17 +2315,17 @@ GeckoDriver.prototype.getElementText = a
  *     If element represented by reference <var>id</var> is unknown.
  * @throws {NoSuchWindowError}
  *     Top-level browsing context has been discarded.
  * @throws {UnexpectedAlertOpenError}
  *     A modal dialog is open, blocking this operation.
  */
 GeckoDriver.prototype.getElementTagName = async function(cmd) {
   assert.open(this.getCurrentWindow());
-  this._assertAndDismissModal();
+  this._handleUserPrompts();
 
   let id = assert.string(cmd.parameters.id);
   let webEl = WebElement.fromUUID(id, this.context);
 
   switch (this.context) {
     case Context.Chrome:
       let el = this.curBrowser.seenEls.get(webEl);
       return el.tagName.toLowerCase();
@@ -2351,17 +2353,17 @@ GeckoDriver.prototype.getElementTagName 
  *     If element represented by reference <var>id</var> is unknown.
  * @throws {NoSuchWindowError}
  *     Top-level browsing context has been discarded.
  * @throws {UnexpectedAlertOpenError}
  *     A modal dialog is open, blocking this operation.
  */
 GeckoDriver.prototype.isElementDisplayed = async function(cmd) {
   assert.open(this.getCurrentWindow());
-  this._assertAndDismissModal();
+  this._handleUserPrompts();
 
   let id = assert.string(cmd.parameters.id);
   let webEl = WebElement.fromUUID(id, this.context);
 
   switch (this.context) {
     case Context.Chrome:
       let el = this.curBrowser.seenEls.get(webEl);
       return interaction.isElementDisplayed(el, this.a11yChecks);
@@ -2391,17 +2393,17 @@ GeckoDriver.prototype.isElementDisplayed
  *     If element represented by reference <var>id</var> is unknown.
  * @throws {NoSuchWindowError}
  *     Top-level browsing context has been discarded.
  * @throws {UnexpectedAlertOpenError}
  *     A modal dialog is open, blocking this operation.
  */
 GeckoDriver.prototype.getElementValueOfCssProperty = async function(cmd) {
   const win = assert.open(this.getCurrentWindow());
-  this._assertAndDismissModal();
+  this._handleUserPrompts();
 
   let id = assert.string(cmd.parameters.id);
   let prop = assert.string(cmd.parameters.propertyName);
   let webEl = WebElement.fromUUID(id, this.context);
 
   switch (this.context) {
     case Context.Chrome:
       let el = this.curBrowser.seenEls.get(webEl);
@@ -2431,17 +2433,17 @@ GeckoDriver.prototype.getElementValueOfC
  *     If element represented by reference <var>id</var> is unknown.
  * @throws {NoSuchWindowError}
  *     Top-level browsing context has been discarded.
  * @throws {UnexpectedAlertOpenError}
  *     A modal dialog is open, blocking this operation.
  */
 GeckoDriver.prototype.isElementEnabled = async function(cmd) {
   assert.open(this.getCurrentWindow());
-  this._assertAndDismissModal();
+  this._handleUserPrompts();
 
   let id = assert.string(cmd.parameters.id);
   let webEl = WebElement.fromUUID(id, this.context);
 
   switch (this.context) {
     case Context.Chrome:
       // Selenium atom doesn't quite work here
       let el = this.curBrowser.seenEls.get(webEl);
@@ -2470,17 +2472,17 @@ GeckoDriver.prototype.isElementEnabled =
  *     If element represented by reference <var>id</var> is unknown.
  * @throws {NoSuchWindowError}
  *     Top-level browsing context has been discarded.
  * @throws {UnexpectedAlertOpenError}
  *     A modal dialog is open, blocking this operation.
  */
 GeckoDriver.prototype.isElementSelected = async function(cmd) {
   assert.open(this.getCurrentWindow());
-  this._assertAndDismissModal();
+  this._handleUserPrompts();
 
   let id = assert.string(cmd.parameters.id);
   let webEl = WebElement.fromUUID(id, this.context);
 
   switch (this.context) {
     case Context.Chrome:
       // Selenium atom doesn't quite work here
       let el = this.curBrowser.seenEls.get(webEl);
@@ -2501,17 +2503,17 @@ GeckoDriver.prototype.isElementSelected 
  *     If element represented by reference <var>id</var> is unknown.
  * @throws {NoSuchWindowError}
  *     Top-level browsing context has been discarded.
  * @throws {UnexpectedAlertOpenError}
  *     A modal dialog is open, blocking this operation.
  */
 GeckoDriver.prototype.getElementRect = async function(cmd) {
   const win = assert.open(this.getCurrentWindow());
-  this._assertAndDismissModal();
+  this._handleUserPrompts();
 
   let id = assert.string(cmd.parameters.id);
   let webEl = WebElement.fromUUID(id, this.context);
 
   switch (this.context) {
     case Context.Chrome:
       let el = this.curBrowser.seenEls.get(webEl);
       let rect = el.getBoundingClientRect();
@@ -2544,17 +2546,17 @@ GeckoDriver.prototype.getElementRect = a
  *     If element represented by reference <var>id</var> is unknown.
  * @throws {NoSuchWindowError}
  *     Top-level browsing context has been discarded.
  * @throws {UnexpectedAlertOpenError}
  *     A modal dialog is open, blocking this operation.
  */
 GeckoDriver.prototype.sendKeysToElement = async function(cmd) {
   assert.open(this.getCurrentWindow());
-  this._assertAndDismissModal();
+  this._handleUserPrompts();
 
   let id = assert.string(cmd.parameters.id);
   let text = assert.string(cmd.parameters.text);
   let webEl = WebElement.fromUUID(id, this.context);
 
   switch (this.context) {
     case Context.Chrome:
       let el = this.curBrowser.seenEls.get(webEl);
@@ -2582,17 +2584,17 @@ GeckoDriver.prototype.sendKeysToElement 
  *     If element represented by reference <var>id</var> is unknown.
  * @throws {NoSuchWindowError}
  *     Top-level browsing context has been discarded.
  * @throws {UnexpectedAlertOpenError}
  *     A modal dialog is open, blocking this operation.
  */
 GeckoDriver.prototype.clearElement = async function(cmd) {
   assert.open(this.getCurrentWindow());
-  this._assertAndDismissModal();
+  this._handleUserPrompts();
 
   let id = assert.string(cmd.parameters.id);
   let webEl = WebElement.fromUUID(id, this.context);
 
   switch (this.context) {
     case Context.Chrome:
       // the selenium atom doesn't work here
       let el = this.curBrowser.seenEls.get(webEl);
@@ -2651,17 +2653,17 @@ GeckoDriver.prototype.switchToShadowRoot
  *     A modal dialog is open, blocking this operation.
  * @throws {InvalidCookieDomainError}
  *     If <var>cookie</var> is for a different domain than the active
  *     document's host.
  */
 GeckoDriver.prototype.addCookie = function(cmd) {
   assert.content(this.context);
   assert.open(this.getCurrentWindow());
-  this._assertAndDismissModal();
+  this._handleUserPrompts();
 
   let {protocol, hostname} = this.currentURL;
 
   const networkSchemes = ["ftp:", "http:", "https:"];
   if (!networkSchemes.includes(protocol)) {
     throw new InvalidCookieDomainError("Document is cookie-averse");
   }
 
@@ -2681,17 +2683,17 @@ GeckoDriver.prototype.addCookie = functi
  * @throws {NoSuchWindowError}
  *     Top-level browsing context has been discarded.
  * @throws {UnexpectedAlertOpenError}
  *     A modal dialog is open, blocking this operation.
  */
 GeckoDriver.prototype.getCookies = function() {
   assert.content(this.context);
   assert.open(this.getCurrentWindow());
-  this._assertAndDismissModal();
+  this._handleUserPrompts();
 
   let {hostname, pathname} = this.currentURL;
   return [...cookie.iter(hostname, pathname)];
 };
 
 /**
  * Delete all cookies that are visible to a document.
  *
@@ -2700,17 +2702,17 @@ GeckoDriver.prototype.getCookies = funct
  * @throws {NoSuchWindowError}
  *     Top-level browsing context has been discarded.
  * @throws {UnexpectedAlertOpenError}
  *     A modal dialog is open, blocking this operation.
  */
 GeckoDriver.prototype.deleteAllCookies = function() {
   assert.content(this.context);
   assert.open(this.getCurrentWindow());
-  this._assertAndDismissModal();
+  this._handleUserPrompts();
 
   let {hostname, pathname} = this.currentURL;
   for (let toDelete of cookie.iter(hostname, pathname)) {
     cookie.remove(toDelete);
   }
 };
 
 /**
@@ -2721,17 +2723,17 @@ GeckoDriver.prototype.deleteAllCookies =
  * @throws {NoSuchWindowError}
  *     Top-level browsing context has been discarded.
  * @throws {UnexpectedAlertOpenError}
  *     A modal dialog is open, blocking this operation.
  */
 GeckoDriver.prototype.deleteCookie = function(cmd) {
   assert.content(this.context);
   assert.open(this.getCurrentWindow());
-  this._assertAndDismissModal();
+  this._handleUserPrompts();
 
   let {hostname, pathname} = this.currentURL;
   let name = assert.string(cmd.parameters.name);
   for (let c of cookie.iter(hostname, pathname)) {
     if (c.name === name) {
       cookie.remove(c);
     }
   }
@@ -2751,17 +2753,17 @@ GeckoDriver.prototype.deleteCookie = fun
  *
  * @throws {NoSuchWindowError}
  *     Top-level browsing context has been discarded.
  * @throws {UnexpectedAlertOpenError}
  *     A modal dialog is open, blocking this operation.
  */
 GeckoDriver.prototype.close = async function() {
   assert.open(this.getCurrentWindow(Context.Content));
-  this._assertAndDismissModal();
+  this._handleUserPrompts();
 
   let nwins = 0;
 
   for (let win of this.windows) {
     // For browser windows count the tabs. Otherwise take the window itself.
     let tabbrowser = browser.getTabBrowser(win);
     if (tabbrowser && tabbrowser.tabs) {
       nwins += tabbrowser.tabs.length;
@@ -3004,17 +3006,17 @@ GeckoDriver.prototype.setScreenOrientati
  * @throws {NoSuchWindowError}
  *     Top-level browsing context has been discarded.
  * @throws {UnexpectedAlertOpenError}
  *     A modal dialog is open, blocking this operation.
  */
 GeckoDriver.prototype.minimizeWindow = async function() {
   assert.firefox();
   const win = assert.open(this.getCurrentWindow());
-  this._assertAndDismissModal();
+  this._handleUserPrompts();
 
   if (WindowState.from(win.windowState) == WindowState.Fullscreen) {
     await exitFullscreen(win);
   }
 
   if (WindowState.from(win.windowState) != WindowState.Minimized) {
     await new Promise(resolve => {
       this.curBrowser.eventObserver.addEventListener("visibilitychange", resolve, {once: true});
@@ -3041,17 +3043,17 @@ GeckoDriver.prototype.minimizeWindow = a
  * @throws {NoSuchWindowError}
  *     Top-level browsing context has been discarded.
  * @throws {UnexpectedAlertOpenError}
  *     A modal dialog is open, blocking this operation.
  */
 GeckoDriver.prototype.maximizeWindow = async function() {
   assert.firefox();
   const win = assert.open(this.getCurrentWindow());
-  this._assertAndDismissModal();
+  this._handleUserPrompts();
 
   switch (WindowState.from(win.windowState)) {
     case WindowState.Fullscreen:
       await exitFullscreen(win);
       break;
 
     case WindowState.Minimized:
       await restoreWindow(win, this.curBrowser.eventObserver);
@@ -3127,17 +3129,17 @@ GeckoDriver.prototype.maximizeWindow = a
  * @throws {NoSuchWindowError}
  *     Top-level browsing context has been discarded.
  * @throws {UnexpectedAlertOpenError}
  *     A modal dialog is open, blocking this operation.
  */
 GeckoDriver.prototype.fullscreenWindow = async function() {
   assert.firefox();
   const win = assert.open(this.getCurrentWindow());
-  this._assertAndDismissModal();
+  this._handleUserPrompts();
 
   if (WindowState.from(win.windowState) == WindowState.Minimized) {
     await restoreWindow(win, this.curBrowser.eventObserver);
   }
 
   if (WindowState.from(win.windowState) != WindowState.Fullscreen) {
     await new Promise(resolve => {
       win.addEventListener("sizemodechange", resolve, {once: true});
@@ -3211,26 +3213,48 @@ GeckoDriver.prototype.sendKeysToDialog =
   // see toolkit/components/prompts/content/commonDialog.js
   let {loginTextbox} = this.dialog.ui;
   await interaction.sendKeysToElement(
       loginTextbox, cmd.parameters.text, this.a11yChecks);
 };
 
 GeckoDriver.prototype._checkIfAlertIsPresent = function() {
   if (!this.dialog || !this.dialog.ui) {
-    throw new NoSuchAlertError("No modal dialog is currently open");
+    throw new NoSuchAlertError();
   }
 };
 
-GeckoDriver.prototype._assertAndDismissModal = function() {
-  try {
-    assert.noUserPrompt(this.dialog);
-  } catch (e) {
-    this.dismissDialog();
-    throw e;
+GeckoDriver.prototype._handleUserPrompts = function() {
+  if (!this.dialog || !this.dialog.ui) {
+    return;
+  }
+
+  let behavior = this.capabilities.get("unhandledPromptBehavior");
+  switch (behavior) {
+    case UnhandledPromptBehavior.Accept:
+      this.acceptDialog();
+      break;
+
+    case UnhandledPromptBehavior.AcceptAndNotify:
+      this.acceptDialog();
+      throw new UnexpectedAlertOpenError();
+
+    case UnhandledPromptBehavior.Dismiss:
+      this.dismissDialog();
+      break;
+
+    case UnhandledPromptBehavior.DismissAndNotify:
+      this.dismissDialog();
+      throw new UnexpectedAlertOpenError();
+
+    case UnhandledPromptBehavior.Ignore:
+      throw new UnexpectedAlertOpenError();
+
+    default:
+      throw new TypeError(`Unknown unhandledPromptBehavior "${behavior}"`);
   }
 };
 
 /**
  * Enables or disables accepting new socket connections.
  *
  * By calling this method with `false` the server will not accept any
  * further connections, but existing connections will not be forcible

--- a/testing/marionette/harness/marionette_harness/tests/unit/test_capabilities.py
+++ b/testing/marionette/harness/marionette_harness/tests/unit/test_capabilities.py
@@ -155,8 +155,38 @@ class TestCapabilityMatching(MarionetteT
 
     def test_timeouts(self):
         timeouts = {u"implicit": 123, u"pageLoad": 456, u"script": 789}
         caps = {"timeouts": timeouts}
         self.marionette.start_session(caps)
         self.assertIn("timeouts", self.marionette.session_capabilities)
         self.assertDictEqual(self.marionette.session_capabilities["timeouts"], timeouts)
         self.assertDictEqual(self.marionette._send_message("getTimeouts"), timeouts)
+
+    def test_unhandled_prompt_behavior(self):
+        behaviors = [
+            "accept",
+            "accept and notify",
+            "dismiss",
+            "dismiss and notify",
+            "ignore"
+        ]
+
+        for behavior in behaviors:
+            print("valid unhandled prompt behavior {}".format(behavior))
+            self.delete_session()
+            self.marionette.start_session({"unhandledPromptBehavior": behavior})
+            self.assertEqual(self.marionette.session_capabilities["unhandledPromptBehavior"],
+                             behavior)
+
+        # Default value
+        self.delete_session()
+        self.marionette.start_session()
+        self.assertEqual(self.marionette.session_capabilities["unhandledPromptBehavior"],
+                         "dismiss and notify")
+
+        # Invalid values
+        self.delete_session()
+        for behavior in [None, "", "ACCEPT", True, 42, {}, []]:
+            print("invalid unhandled prompt behavior {}".format(behavior))
+            with self.assertRaisesRegexp(SessionNotCreatedException, "InvalidArgumentError"):
+                self.marionette.start_session({"unhandledPromptBehavior": behavior})
+

--- a/testing/marionette/harness/marionette_harness/tests/unit/test_modal_dialogs.py
+++ b/testing/marionette/harness/marionette_harness/tests/unit/test_modal_dialogs.py
@@ -10,40 +10,42 @@ from marionette_driver import errors
 from marionette_driver.marionette import Alert
 from marionette_driver.wait import Wait
 
 from marionette_harness import MarionetteTestCase, WindowManagerMixin
 
 
 class BaseAlertTestCase(WindowManagerMixin, MarionetteTestCase):
 
+    @property
     def alert_present(self):
         try:
             Alert(self.marionette).text
             return True
         except errors.NoAlertPresentException:
             return False
 
     def wait_for_alert(self, timeout=None):
         Wait(self.marionette, timeout=timeout).until(
-            lambda _: self.alert_present())
+            lambda _: self.alert_present)
 
     def wait_for_alert_closed(self, timeout=None):
         Wait(self.marionette, timeout=timeout).until(
-            lambda _: not self.alert_present())
+            lambda _: not self.alert_present)
 
 
 class TestTabModalAlerts(BaseAlertTestCase):
 
     def setUp(self):
         super(TestTabModalAlerts, self).setUp()
         self.assertTrue(self.marionette.get_pref("prompts.tab_modal.enabled",
                         "Tab modal alerts should be enabled by default."))
 
-        self.marionette.navigate(self.marionette.absolute_url("test_tab_modal_dialogs.html"))
+        self.test_page = self.marionette.absolute_url("test_tab_modal_dialogs.html")
+        self.marionette.navigate(self.test_page)
 
     def tearDown(self):
         # Ensure to close a possible remaining tab modal dialog
         try:
             alert = self.marionette.switch_to_alert()
             alert.dismiss()
 
             self.wait_for_alert_closed()
@@ -178,17 +180,17 @@ class TestTabModalAlerts(BaseAlertTestCa
             self.marionette.find_element(By.ID, "click-result")
 
     def test_modal_is_dismissed_after_unexpected_alert(self):
         self.marionette.find_element(By.ID, "tab-modal-alert").click()
         self.wait_for_alert()
         with self.assertRaises(errors.UnexpectedAlertOpen):
             self.marionette.find_element(By.ID, "click-result")
 
-        assert not self.alert_present()
+        assert not self.alert_present
 
 
 class TestModalAlerts(BaseAlertTestCase):
 
     def setUp(self):
         super(TestModalAlerts, self).setUp()
         self.marionette.set_pref("network.auth.non-web-content-triggered-resources-http-auth-allow",
                                  True)

new file mode 100644
--- /dev/null
+++ b/testing/marionette/harness/marionette_harness/tests/unit/test_unhandled_prompt_behavior.py
@@ -0,0 +1,104 @@
+from __future__ import absolute_import
+
+from marionette_driver import errors
+from marionette_driver.marionette import Alert
+from marionette_driver.wait import Wait
+from marionette_harness import MarionetteTestCase, parameterized
+
+
+class TestUnhandledPromptBehavior(MarionetteTestCase):
+
+    def setUp(self):
+        super(TestUnhandledPromptBehavior, self).setUp()
+
+        self.marionette.delete_session()
+
+    def tearDown(self):
+        # Ensure to close a possible remaining tab modal dialog
+        try:
+            alert = self.marionette.switch_to_alert()
+            alert.dismiss()
+
+            Wait(self.marionette).until(lambda _: not self.alert_present)
+        except errors.NoAlertPresentException:
+            pass
+
+        super(TestUnhandledPromptBehavior, self).tearDown()
+
+    @property
+    def alert_present(self):
+        try:
+            Alert(self.marionette).text
+            return True
+        except errors.NoAlertPresentException:
+            return False
+
+    def perform_user_prompt_check(self, prompt_type, text, expected_result,
+                                  expected_close=True, expected_notify=True):
+        if prompt_type not in ["alert", "confirm", "prompt"]:
+            raise TypeError("Invalid dialog type: {}".format(prompt_type))
+
+        # No need to call resolve() because opening a prompt stops the script
+        self.marionette.execute_async_script("""
+            window.return_value = null;
+            window.return_value = window[arguments[0]](arguments[1]);
+        """, script_args=(prompt_type, text))
+
+        if expected_notify:
+            with self.assertRaises(errors.UnexpectedAlertOpen):
+                self.marionette.title
+            # Bug 1469752 - WebDriverError misses optional data property
+            # self.assertEqual(ex.data.text, text)
+        else:
+            self.marionette.title
+
+        self.assertEqual(self.alert_present, not expected_close)
+
+        prompt_result = self.marionette.execute_script(
+            "return window.return_value", new_sandbox=False)
+        self.assertEqual(prompt_result, expected_result)
+
+    @parameterized("alert", "alert", None)
+    @parameterized("confirm", "confirm", True)
+    @parameterized("prompt", "prompt", "")
+    def test_accept(self, prompt_type, result):
+        self.marionette.start_session({"unhandledPromptBehavior": "accept"})
+        self.perform_user_prompt_check(prompt_type, "foo {}".format(prompt_type), result,
+                                       expected_notify=False)
+
+    @parameterized("alert", "alert", None)
+    @parameterized("confirm", "confirm", True)
+    @parameterized("prompt", "prompt", "")
+    def test_accept_and_notify(self, prompt_type, result):
+        self.marionette.start_session({"unhandledPromptBehavior": "accept and notify"})
+        self.perform_user_prompt_check(prompt_type, "foo {}".format(prompt_type), result)
+
+    @parameterized("alert", "alert", None)
+    @parameterized("confirm", "confirm", False)
+    @parameterized("prompt", "prompt", None)
+    def test_dismiss(self, prompt_type, result):
+        self.marionette.start_session({"unhandledPromptBehavior": "dismiss"})
+        self.perform_user_prompt_check(prompt_type, "foo {}".format(prompt_type), result,
+                                       expected_notify=False)
+
+    @parameterized("alert", "alert", None)
+    @parameterized("confirm", "confirm", False)
+    @parameterized("prompt", "prompt", None)
+    def test_dismiss_and_notify(self, prompt_type, result):
+        self.marionette.start_session({"unhandledPromptBehavior": "dismiss and notify"})
+        self.perform_user_prompt_check(prompt_type, "foo {}".format(prompt_type), result)
+
+    @parameterized("alert", "alert", None)
+    @parameterized("confirm", "confirm", None)
+    @parameterized("prompt", "prompt", None)
+    def test_ignore(self, prompt_type, result):
+        self.marionette.start_session({"unhandledPromptBehavior": "ignore"})
+        self.perform_user_prompt_check(prompt_type, "foo {}".format(prompt_type), result,
+                                       expected_close=False)
+
+    @parameterized("alert", "alert", None)
+    @parameterized("confirm", "confirm", False)
+    @parameterized("prompt", "prompt", None)
+    def test_default(self, prompt_type, result):
+        self.marionette.start_session({})
+        self.perform_user_prompt_check(prompt_type, "foo {}".format(prompt_type), result)

--- a/testing/marionette/harness/marionette_harness/tests/unit/unit-tests.ini
+++ b/testing/marionette/harness/marionette_harness/tests/unit/unit-tests.ini
@@ -94,16 +94,19 @@ skip-if = true # Bug 925688
 [test_profile_management.py]
 skip-if = manage_instance == false || appname == 'fennec' # Bug 1298921
 [test_quit_restart.py]
 skip-if = manage_instance == false || appname == 'fennec' # Bug 1298921
 [test_context.py]
 
 [test_modal_dialogs.py]
 skip-if = appname == 'fennec' # Bug 1325738
+[test_unhandled_prompt_behavior.py]
+skip-if = appname == 'fennec' # Bug 1325738
+
 [test_key_actions.py]
 [test_legacy_mouse_action.py]
 skip-if = appname == 'fennec'
 [test_mouse_action.py]
 skip-if = appname == 'fennec'
 [test_teardown_context_preserved.py]
 [test_file_upload.py]
 skip-if = appname == 'fennec' || os == "win" # http://bugs.python.org/issue14574

--- a/testing/marionette/harness/marionette_harness/www/test_tab_modal_dialogs.html
+++ b/testing/marionette/harness/marionette_harness/www/test_tab_modal_dialogs.html
@@ -1,16 +1,16 @@
 <!-- This Source Code Form is subject to the terms of the Mozilla Public
    - License, v. 2.0. If a copy of the MPL was not distributed with this
    - file, You can obtain one at http://mozilla.org/MPL/2.0/. -->
 
 <!DOCTYPE html>
 <html>
 <head>
-  <title>Marionette Test</title>
+  <title>Dialog Test</title>
   <script type="text/javascript">
     function handleAlert () {
       window.alert('Marionette alert');
     }
 
     function handleConfirm () {
       var alertAccepted = window.confirm('Marionette confirm');
       document.getElementById('confirm-result').innerHTML = alertAccepted;

--- a/testing/marionette/test/unit/test_capabilities.js
+++ b/testing/marionette/test/unit/test_capabilities.js
@@ -8,16 +8,17 @@ ChromeUtils.import("resource://gre/modul
 ChromeUtils.import("resource://gre/modules/Services.jsm");
 
 const {InvalidArgumentError} = ChromeUtils.import("chrome://marionette/content/error.js", {});
 const {
   Capabilities,
   PageLoadStrategy,
   Proxy,
   Timeouts,
+  UnhandledPromptBehavior,
 } = ChromeUtils.import("chrome://marionette/content/capabilities.js", {});
 
 add_test(function test_Timeouts_ctor() {
   let ts = new Timeouts();
   equal(ts.implicit, 0);
   equal(ts.pageLoad, 300000);
   equal(ts.script, 30000);
 
@@ -361,16 +362,26 @@ add_test(function test_Proxy_fromJSON() 
   p.proxyType = "manual";
   p.noProxy = ["2001:db8::1"];
   let manual = {proxyType: "manual", "noProxy": ["[2001:db8::1]"]};
   deepEqual(p, Proxy.fromJSON(manual));
 
   run_next_test();
 });
 
+add_test(function test_UnhandledPromptBehavior() {
+  equal(UnhandledPromptBehavior.Accept, "accept");
+  equal(UnhandledPromptBehavior.AcceptAndNotify, "accept and notify");
+  equal(UnhandledPromptBehavior.Dismiss, "dismiss");
+  equal(UnhandledPromptBehavior.DismissAndNotify, "dismiss and notify");
+  equal(UnhandledPromptBehavior.Ignore, "ignore");
+
+  run_next_test();
+});
+
 add_test(function test_Capabilities_ctor() {
   let caps = new Capabilities();
   ok(caps.has("browserName"));
   ok(caps.has("browserVersion"));
   ok(caps.has("platformName"));
   ok(["linux", "mac", "windows", "android"].includes(caps.get("platformName")));
   ok(caps.has("platformVersion"));
   equal(PageLoadStrategy.Normal, caps.get("pageLoadStrategy"));

deleted file mode 100644
--- a/testing/web-platform/meta/webdriver/tests/close_window/user_prompts.py.ini
+++ /dev/null
@@ -1,8 +0,0 @@
-[user_prompts.py]
-  [test_handle_prompt_accept[capabilities0-alert\]]
-    expected: FAIL
-  [test_handle_prompt_accept[capabilities0-confirm\]]
-    expected: FAIL
-  [test_handle_prompt_accept[capabilities0-prompt\]]
-    expected: FAIL
-

deleted file mode 100644
--- a/testing/web-platform/meta/webdriver/tests/delete_cookie/user_prompts.py.ini
+++ /dev/null
@@ -1,9 +0,0 @@
-[user_prompts.py]
-  [test_handle_prompt_accept[capabilities0-alert\]]
-    expected: FAIL
-
-  [test_handle_prompt_accept[capabilities0-confirm\]]
-    expected: FAIL
-
-  [test_handle_prompt_accept[capabilities0-prompt\]]
-    expected: FAIL

deleted file mode 100644
--- a/testing/web-platform/meta/webdriver/tests/element_send_keys/user_prompts.py.ini
+++ /dev/null
@@ -1,9 +0,0 @@
-[user_prompts.py]
-  [test_handle_prompt_accept[capabilities0-alert\]]
-    expected: FAIL
-
-  [test_handle_prompt_accept[capabilities0-confirm\]]
-    expected: FAIL
-
-  [test_handle_prompt_accept[capabilities0-prompt\]]
-    expected: FAIL

--- a/testing/web-platform/meta/webdriver/tests/execute_async_script/user_prompts.py.ini
+++ b/testing/web-platform/meta/webdriver/tests/execute_async_script/user_prompts.py.ini
@@ -1,38 +1,11 @@
 [user_prompts.py]
   disabled:
     if webrender: bug 1425588
-  [test_handle_prompt_accept[capabilities0-alert\]]
-    expected: FAIL
-
-  [test_handle_prompt_accept[capabilities0-confirm\]]
-    expected: FAIL
-
-  [test_handle_prompt_accept[capabilities0-prompt\]]
-    expected: FAIL
-
-  [test_handle_prompt_dismiss[capabilities0-alert\]]
-    expected: FAIL
-
-  [test_handle_prompt_dismiss[capabilities0-confirm\]]
-    expected: FAIL
-
-  [test_handle_prompt_dismiss[capabilities0-prompt\]]
-    expected: FAIL
-
-  [test_handle_prompt_ignore[capabilities0-alert\]]
-    expected: FAIL
-
-  [test_handle_prompt_ignore[capabilities0-confirm\]]
-    expected: FAIL
-
-  [test_handle_prompt_ignore[capabilities0-prompt\]]
-    expected: FAIL
-
   [test_handle_prompt_twice[capabilities0-alert\]]
     expected: FAIL
     disabled: Bug 1459118
 
   [test_handle_prompt_twice[capabilities0-confirm\]]
     expected: FAIL
     disabled: Bug 1459118
 

--- a/testing/web-platform/meta/webdriver/tests/execute_script/user_prompts.py.ini
+++ b/testing/web-platform/meta/webdriver/tests/execute_script/user_prompts.py.ini
@@ -1,38 +1,11 @@
 [user_prompts.py]
   disabled:
     if webrender: bug 1425588
-  [test_handle_prompt_accept[capabilities0-alert\]]
-    expected: FAIL
-
-  [test_handle_prompt_accept[capabilities0-confirm\]]
-    expected: FAIL
-
-  [test_handle_prompt_accept[capabilities0-prompt\]]
-    expected: FAIL
-
-  [test_handle_prompt_dismiss[capabilities0-alert\]]
-    expected: FAIL
-
-  [test_handle_prompt_dismiss[capabilities0-confirm\]]
-    expected: FAIL
-
-  [test_handle_prompt_dismiss[capabilities0-prompt\]]
-    expected: FAIL
-
-  [test_handle_prompt_ignore[capabilities0-alert\]]
-    expected: FAIL
-
-  [test_handle_prompt_ignore[capabilities0-confirm\]]
-    expected: FAIL
-
-  [test_handle_prompt_ignore[capabilities0-prompt\]]
-    expected: FAIL
-
   [test_handle_prompt_twice[capabilities0-alert\]]
     expected: FAIL
     disabled: Bug 1459118
 
   [test_handle_prompt_twice[capabilities0-confirm\]]
     expected: FAIL
     disabled: Bug 1459118
 

deleted file mode 100644
--- a/testing/web-platform/meta/webdriver/tests/fullscreen_window/user_prompts.py.ini
+++ /dev/null
@@ -1,9 +0,0 @@
-[user_prompts.py]
-  [test_handle_prompt_accept[capabilities0-alert\]]
-    expected: FAIL
-
-  [test_handle_prompt_accept[capabilities0-confirm\]]
-    expected: FAIL
-
-  [test_handle_prompt_accept[capabilities0-prompt\]]
-    expected: FAIL

deleted file mode 100644
--- a/testing/web-platform/meta/webdriver/tests/get_current_url/user_prompts.py.ini
+++ /dev/null
@@ -1,9 +0,0 @@
-[user_prompts.py]
-  [test_handle_prompt_accept[capabilities0-alert\]]
-    expected: FAIL
-
-  [test_handle_prompt_accept[capabilities0-confirm\]]
-    expected: FAIL
-
-  [test_handle_prompt_accept[capabilities0-prompt\]]
-    expected: FAIL

--- a/testing/web-platform/meta/webdriver/tests/get_element_property/user_prompts.py.ini
+++ b/testing/web-platform/meta/webdriver/tests/get_element_property/user_prompts.py.ini
@@ -1,20 +1,3 @@
 [user_prompts.py]
   disabled:
     if webrender: bug 1425588
-  [test_handle_prompt_accept[capabilities0-alert\]]
-    expected: FAIL
-
-  [test_handle_prompt_accept[capabilities0-confirm\]]
-    expected: FAIL
-
-  [test_handle_prompt_accept[capabilities0-prompt\]]
-    expected: FAIL
-
-  [test_handle_prompt_dismiss[capabilities0-alert\]]
-    expected: FAIL
-
-  [test_handle_prompt_dismiss[capabilities0-confirm\]]
-    expected: FAIL
-
-  [test_handle_prompt_dismiss[capabilities0-prompt\]]
-    expected: FAIL

--- a/testing/web-platform/meta/webdriver/tests/get_element_tag_name/user_prompts.py.ini
+++ b/testing/web-platform/meta/webdriver/tests/get_element_tag_name/user_prompts.py.ini
@@ -1,20 +1,3 @@
 [user_prompts.py]
   disabled:
     if webrender: bug 1425588
-  [test_handle_prompt_accept[capabilities0-alert\]]
-    expected: FAIL
-
-  [test_handle_prompt_accept[capabilities0-confirm\]]
-    expected: FAIL
-
-  [test_handle_prompt_accept[capabilities0-prompt\]]
-    expected: FAIL
-
-  [test_handle_prompt_dismiss[capabilities0-alert\]]
-    expected: FAIL
-
-  [test_handle_prompt_dismiss[capabilities0-confirm\]]
-    expected: FAIL
-
-  [test_handle_prompt_dismiss[capabilities0-prompt\]]
-    expected: FAIL

--- a/testing/web-platform/meta/webdriver/tests/get_title/user_prompts.py.ini
+++ b/testing/web-platform/meta/webdriver/tests/get_title/user_prompts.py.ini
@@ -1,20 +1,3 @@
 [user_prompts.py]
   disabled:
     if webrender: bug 1425588
-  [test_handle_prompt_accept[capabilities0-alert\]]
-    expected: FAIL
-
-  [test_handle_prompt_accept[capabilities0-confirm\]]
-    expected: FAIL
-
-  [test_handle_prompt_accept[capabilities0-prompt\]]
-    expected: FAIL
-
-  [test_handle_prompt_dismiss[capabilities0-alert\]]
-    expected: FAIL
-
-  [test_handle_prompt_dismiss[capabilities0-confirm\]]
-    expected: FAIL
-
-  [test_handle_prompt_dismiss[capabilities0-prompt\]]
-    expected: FAIL

--- a/testing/web-platform/meta/webdriver/tests/get_window_rect/user_prompts.py.ini
+++ b/testing/web-platform/meta/webdriver/tests/get_window_rect/user_prompts.py.ini
@@ -1,11 +1,3 @@
 [user_prompts.py]
   disabled:
     if not debug and e10s and (os == "linux") and (version == "Ubuntu 16.04") and (processor == "x86_64") and (bits == 64): wpt-sync Bug 1446953
-  [test_handle_prompt_accept[capabilities0-alert\]]
-    expected: FAIL
-
-  [test_handle_prompt_accept[capabilities0-confirm\]]
-    expected: FAIL
-
-  [test_handle_prompt_accept[capabilities0-prompt\]]
-    expected: FAIL

deleted file mode 100644
--- a/testing/web-platform/meta/webdriver/tests/is_element_selected/user_prompts.py.ini
+++ /dev/null
@@ -1,18 +0,0 @@
-[user_prompts.py]
-  [test_handle_prompt_dismiss[capabilities0-alert\]]
-    expected: FAIL
-
-  [test_handle_prompt_dismiss[capabilities0-confirm\]]
-    expected: FAIL
-
-  [test_handle_prompt_dismiss[capabilities0-prompt\]]
-    expected: FAIL
-
-  [test_handle_prompt_accept[capabilities0-alert\]]
-    expected: FAIL
-
-  [test_handle_prompt_accept[capabilities0-confirm\]]
-    expected: FAIL
-
-  [test_handle_prompt_accept[capabilities0-prompt\]]
-    expected: FAIL

--- a/testing/web-platform/meta/webdriver/tests/minimize_window/user_prompts.py.ini
+++ b/testing/web-platform/meta/webdriver/tests/minimize_window/user_prompts.py.ini
@@ -1,12 +1,3 @@
 [user_prompts.py]
   disabled:
     if not debug and not webrender and e10s and (os == "linux") and (version == "Ubuntu 16.04") and (processor == "x86") and (bits == 32): wpt-sync Bug 1449780
-  [test_handle_prompt_accept[capabilities0-alert\]]
-    expected: FAIL
-
-  [test_handle_prompt_accept[capabilities0-confirm\]]
-    expected: FAIL
-
-  [test_handle_prompt_accept[capabilities0-prompt\]]
-    expected: FAIL
-

deleted file mode 100644
--- a/testing/web-platform/meta/webdriver/tests/set_window_rect/user_prompts.py.ini
+++ /dev/null
@@ -1,10 +0,0 @@
-[user_prompts.py]
-  [test_handle_prompt_accept[capabilities0-alert\]]
-    expected: FAIL
-
-  [test_handle_prompt_accept[capabilities0-confirm\]]
-    expected: FAIL
-
-  [test_handle_prompt_accept[capabilities0-prompt\]]
-    expected: FAIL
-