--- a/testing/marionette/assert.js
+++ b/testing/marionette/assert.js
@@ -137,17 +137,34 @@ assert.window = function (win, msg = "")
     try {
       return w && w.document.defaultView;
 
     // If the window is no longer available a TypeError is thrown.
     } catch (e if e.name === "TypeError") {
       return null;
     }
   }, msg, NoSuchWindowError)(win);
-}
+};
+
+/**
+ * Asserts that there is no current user prompt.
+ *
+ * @param {modal.Dialog} dialog
+ *     Reference to current dialogue.
+ * @param {string=} msg
+ *     Custom error message.
+ *
+ * @throws {UnexpectedAlertOpenError}
+ *     If there is a user prompt.
+ */
+assert.noUserPrompt = function (dialog, msg = "") {
+  assert.that(d => d === null || typeof d == "undefined",
+      msg,
+      UnexpectedAlertOpenError)(dialog);
+};
 
 /**
  * Asserts that |obj| is defined.
  *
  * @param {?} obj
  *     Value to test.
  * @param {string=} msg
  *     Custom error message.

--- a/testing/marionette/driver.js
+++ b/testing/marionette/driver.js
@@ -966,20 +966,28 @@ GeckoDriver.prototype.executeJSScript = 
  * current window triggers and document.readyState is "complete".
  *
  * In chrome context it will change the current window's location to
  * the supplied URL and wait until document.readyState equals "complete"
  * or the page timeout duration has elapsed.
  *
  * @param {string} url
  *     URL to navigate to.
+ *
+ * @throws {UnsupportedOperationError}
+ *     Not available in current context.
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
  */
 GeckoDriver.prototype.get = function* (cmd, resp) {
   assert.content(this.context);
   assert.window(this.getCurrentWindow());
+  assert.noUserPrompt(this.dialog);
 
   let url = cmd.parameters.url;
 
   let get = this.listener.get({url: url, pageTimeout: this.timeouts.pageLoad});
 
   // If a remoteness update interrupts our page load, this will never return
   // We need to re-issue this request to correctly poll for readyState and
   // send errors.
@@ -1004,32 +1012,49 @@ GeckoDriver.prototype.get = function* (c
  * Get a string representing the current URL.
  *
  * On Desktop this returns a string representation of the URL of the
  * current top level browsing context.  This is equivalent to
  * document.location.href.
  *
  * When in the context of the chrome, this returns the canonical URL
  * of the current resource.
+ *
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
  */
 GeckoDriver.prototype.getCurrentUrl = function (cmd) {
-  let win = assert.window(this.getCurrentWindow());
+  const win = assert.window(this.getCurrentWindow());
+  assert.noUserPrompt(this.dialog);
 
   switch (this.context) {
     case Context.CHROME:
       return win.location.href;
 
     case Context.CONTENT:
       return this.listener.getCurrentUrl();
   }
 };
 
-/** Gets the current title of the window. */
+/**
+ * Gets the current title of the window.
+ *
+ * @return {string}
+ *     Document title of the top-level browsing context.
+ *
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
+ */
 GeckoDriver.prototype.getTitle = function* (cmd, resp) {
-  let win = assert.window(this.getCurrentWindow());
+  const win = assert.window(this.getCurrentWindow());
+  assert.noUserPrompt(this.dialog);
 
   switch (this.context) {
     case Context.CHROME:
       resp.body.value = win.document.documentElement.getAttribute("title");
       break;
 
     case Context.CONTENT:
       resp.body.value = yield this.listener.getTitle();
@@ -1039,39 +1064,59 @@ GeckoDriver.prototype.getTitle = functio
 
 /** Gets the current type of the window. */
 GeckoDriver.prototype.getWindowType = function (cmd, resp) {
   let win = assert.window(this.getCurrentWindow());
 
   resp.body.value = win.document.documentElement.getAttribute("windowtype");
 };
 
-/** Gets the page source of the content document. */
+/**
+ * Gets the page source of the content document.
+ *
+ * @return {string}
+ *     String serialisation of the DOM of the current browsing context's
+ *     active document.
+ *
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
+ */
 GeckoDriver.prototype.getPageSource = function* (cmd, resp) {
-  let win = assert.window(this.getCurrentWindow());
+  const win = assert.window(this.getCurrentWindow());
+  assert.noUserPrompt(this.dialog);
 
   switch (this.context) {
     case Context.CHROME:
       let s = new win.XMLSerializer();
       resp.body.value = s.serializeToString(win.document);
       break;
 
     case Context.CONTENT:
       resp.body.value = yield this.listener.getPageSource();
       break;
   }
 };
 
 /**
  * Cause the browser to traverse one step backward in the joint history
  * of the current browsing context.
+ *
+ * @throws {UnsupportedOperationError}
+ *     Not available in current context.
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
  */
 GeckoDriver.prototype.goBack = function* (cmd, resp) {
   assert.content(this.context);
   assert.window(this.getCurrentWindow());
+  assert.noUserPrompt(this.dialog);
 
   if (!this.curBrowser.tab) {
     // Navigation does not work for non-browser windows
     return;
   }
 
   if (!this.curBrowser.contentBrowser.webNavigation.canGoBack) {
     return;
@@ -1097,20 +1142,28 @@ GeckoDriver.prototype.goBack = function*
   });
 
   yield goBack;
 };
 
 /**
  * Cause the browser to traverse one step forward in the joint history
  * of the current browsing context.
+ *
+ * @throws {UnsupportedOperationError}
+ *     Not available in current context.
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
  */
 GeckoDriver.prototype.goForward = function* (cmd, resp) {
   assert.content(this.context);
   assert.window(this.getCurrentWindow());
+  assert.noUserPrompt(this.dialog);
 
   if (!this.curBrowser.tab) {
     // Navigation does not work for non-browser windows
     return;
   }
 
   if (!this.curBrowser.contentBrowser.webNavigation.canGoForward) {
     return;
@@ -1134,21 +1187,30 @@ GeckoDriver.prototype.goForward = functi
         "Marionette:waitForPageLoaded" + this.curBrowser.curFrameId,
         parameters);
   });
 
   yield goForward;
 };
 
 /**
- * Causes the browser to reload the page in in current top-level browsing context.
+ * Causes the browser to reload the page in current top-level browsing
+ * context.
+ *
+ * @throws {UnsupportedOperationError}
+ *     Not available in current context.
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
  */
 GeckoDriver.prototype.refresh = function* (cmd, resp) {
   assert.content(this.context);
   assert.window(this.getCurrentWindow());
+  assert.noUserPrompt(this.dialog);
 
   yield this.listener.refresh({pageTimeout: this.timeouts.pageLoad});
 };
 
 /**
  * Forces an update for the given browser's id.
  */
 GeckoDriver.prototype.updateIdForBrowser = function (browser, newId) {
@@ -1259,49 +1321,68 @@ GeckoDriver.prototype.getChromeWindowHan
  *
  * Will return the current browser window size in pixels. Refers to
  * window outerWidth and outerHeight values, which include scroll bars,
  * title bars, etc.
  *
  * @return {Object.<string, number>}
  *     Object with |x| and |y| coordinates, and |width| and |height|
  *     of browser window.
+ *
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
  */
 GeckoDriver.prototype.getWindowRect = function (cmd, resp) {
-  let win = assert.window(this.getCurrentWindow());
+  const win = assert.window(this.getCurrentWindow());
+  assert.noUserPrompt(this.dialog);
+
   return {
     x: win.screenX,
     y: win.screenY,
     width: win.outerWidth,
     height: win.outerHeight,
   };
 };
 
 /**
- * Set the window position and size of the browser on the OS Window Manager
+ * Set the window position and size of the browser on the operating
+ * system window manager.
  *
- * The supplied width and height values refer to the window outerWidth
+ * The supplied |width| and |height| values refer to the window outerWidth
  * and outerHeight values, which include browser chrome and OS-level
  * window borders.
+ *
  * @param {number} x
  *     X coordinate of the top/left of the window that it will be
  *     moved to.
  * @param {number} y
  *     Y coordinate of the top/left of the window that it will be
  *     moved to.
+ * @param {number} width
+ *     Width to resize the window to.
+ * @param {number} height
+ *     Height to resize the window to.
  *
  * @return {Object.<string, number>}
- *     Object with |x| and |y| coordinates
- *     and |width| and |height| dimensions
+ *     Object with |x| and |y| coordinates and |width| and |height|
+ *     dimensions.
  *
+ * @throws {UnsupportedOperationError}
+ *     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.setWindowRect = function* (cmd, resp) {
-  assert.firefox()
-
-  let win = assert.window(this.getCurrentWindow());
+  assert.firefox();
+  const win = assert.window(this.getCurrentWindow());
+  assert.noUserPrompt(this.dialog);
 
   let {x, y, height, width} = cmd.parameters;
 
   if (height != null && width != null) {
     assert.positiveInteger(height);
     assert.positiveInteger(width);
     yield new Promise(resolve => {
       // When the DOM resize event claims that it fires _after_ the document
@@ -1446,33 +1527,49 @@ GeckoDriver.prototype.getActiveFrame = f
       if (this.currentFrameElement !== null) {
         let el = element.makeWebElement(this.currentFrameElement);
         resp.body.value = el;
       }
       break;
   }
 };
 
-GeckoDriver.prototype.switchToParentFrame = function*(cmd, resp) {
+/**
+ * Set the current browsing context for future commands to the parent
+ * of the current browsing context.
+ *
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
+ */
+GeckoDriver.prototype.switchToParentFrame = function* (cmd, resp) {
   assert.window(this.getCurrentWindow());
-
-  let res = yield this.listener.switchToParentFrame();
+  assert.noUserPrompt(this.dialog);
+
+  yield this.listener.switchToParentFrame();
 };
 
 /**
  * Switch to a given frame within the current window.
  *
  * @param {Object} element
  *     A web element reference to the element to switch to.
  * @param {(string|number)} id
  *     If element is not defined, then this holds either the id, name,
  *     or index of the frame to switch to.
+ *
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
  */
 GeckoDriver.prototype.switchToFrame = function* (cmd, resp) {
   assert.window(this.getCurrentWindow());
+  assert.noUserPrompt(this.dialog);
 
   let {id, element, focus} = cmd.parameters;
 
   const otherErrorsExpr = /about:.+(error)|(blocked)\?/;
   const checkTimer = Cc["@mozilla.org/timer;1"].createInstance(Ci.nsITimer);
 
   let curWindow = this.getCurrentWindow();
 
@@ -1678,59 +1775,70 @@ GeckoDriver.prototype.singleTap = functi
 
 /**
  * Perform a series of grouped actions at the specified points in time.
  *
  * @param {Array.<?>} actions
  *     Array of objects that each represent an action sequence.
  *
  * @throws {UnsupportedOperationError}
- *     If the command is made in chrome context.
+ *     Not yet available in current context.
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
  */
-GeckoDriver.prototype.performActions = function(cmd, resp) {
+GeckoDriver.prototype.performActions = function (cmd, resp) {
+  assert.content(this.context,
+      "Command 'performActions' is not yet available in chrome context");
   assert.window(this.getCurrentWindow());
-
-  switch (this.context) {
-    case Context.CHROME:
-      throw new UnsupportedOperationError(
-          "Command 'performActions' is not yet available in chrome context");
-
-    case Context.CONTENT:
-      return this.listener.performActions({"actions": cmd.parameters.actions});
-  }
+  assert.noUserPrompt(this.dialog);
+
+  let actions = cmd.parameters.actions;
+  yield this.listener.performActions({"actions": actions});
 };
 
 /**
  * Release all the keys and pointer buttons that are currently depressed.
+ *
+ * @throws {UnsupportedOperationError}
+ *     Not available in current context.
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
  */
 GeckoDriver.prototype.releaseActions = function(cmd, resp) {
+  assert.content(this.context);
   assert.window(this.getCurrentWindow());
-
-  switch (this.context) {
-    case Context.CHROME:
-      throw new UnsupportedOperationError(
-          "Command 'releaseActions' is not yet available in chrome context");
-
-    case Context.CONTENT:
-        return this.listener.releaseActions();
-  }
+  assert.noUserPrompt(this.dialog);
+
+  yield this.listener.releaseActions();
 };
 
 /**
  * An action chain.
  *
  * @param {Object} value
  *     A nested array where the inner array represents each event,
  *     and the outer array represents a collection of events.
  *
  * @return {number}
  *     Last touch ID.
+ *
+ * @throws {UnsupportedOperationError}
+ *     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 = function*(cmd, resp) {
-  let win = assert.window(this.getCurrentWindow());
+  const win = assert.window(this.getCurrentWindow());
+  assert.noUserPrompt(this.dialog);
 
   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();
@@ -1748,42 +1856,51 @@ GeckoDriver.prototype.actionChain = func
 
 /**
  * A multi-action chain.
  *
  * @param {Object} value
  *     A nested array where the inner array represents eache vent,
  *     the middle array represents a collection of events for each
  *     finger, and the outer array represents all fingers.
+ *
+ * @throws {UnsupportedOperationError}
+ *     Not available in current context.
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
  */
-GeckoDriver.prototype.multiAction = function*(cmd, resp) {
+GeckoDriver.prototype.multiAction = function* (cmd, resp) {
+  assert.content(this.context);
   assert.window(this.getCurrentWindow());
-
-  switch (this.context) {
-    case Context.CHROME:
-      throw new UnsupportedOperationError(
-          "Command 'multiAction' is not yet available in chrome context");
-
-    case Context.CONTENT:
-      this.addFrameCloseListener("multi action chain");
-      yield this.listener.multiAction(cmd.parameters.value, cmd.parameters.max_length);
-      break;
-  }
+  assert.noUserPrompt(this.dialog);
+
+  let {value, max_length} = cmd.parameters;
+
+  this.addFrameCloseListener("multi action chain");
+  yield this.listener.multiAction(value, max_length);
 };
 
 /**
  * Find an element using the indicated search strategy.
  *
  * @param {string} using
  *     Indicates which search method to use.
  * @param {string} value
  *     Value the client is looking for.
+ *
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
  */
-GeckoDriver.prototype.findElement = function*(cmd, resp) {
-  let win = assert.window(this.getCurrentWindow());
+GeckoDriver.prototype.findElement = function* (cmd, resp) {
+  const win = assert.window(this.getCurrentWindow());
+  assert.noUserPrompt(this.dialog);
 
   let strategy = cmd.parameters.using;
   let expr = cmd.parameters.value;
   let opts = {
     startNode: cmd.parameters.element,
     timeout: this.timeouts.implicit,
     all: false,
   };
@@ -1854,39 +1971,51 @@ GeckoDriver.prototype.findElements = fun
       resp.body = yield this.listener.findElementsContent(
           cmd.parameters.using,
           cmd.parameters.value,
           opts);
       break;
   }
 };
 
-/** Return the active element on the page. */
-GeckoDriver.prototype.getActiveElement = function*(cmd, resp) {
+/**
+ * Return the active element on the page.
+ *
+ * @return {WebElement}
+ *     Active element of the current browsing context's document element.
+ *
+ * @throws {UnsupportedOperationError}
+ *     Not available in current context.
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
+ */
+GeckoDriver.prototype.getActiveElement = function* (cmd, resp) {
+  assert.content(this.context);
   assert.window(this.getCurrentWindow());
-
-  switch (this.context) {
-    case Context.CHROME:
-      throw new UnsupportedOperationError(
-          "Command 'getActiveElement' is not yet available in chrome context");
-
-    case Context.CONTENT:
-      resp.body.value = yield this.listener.getActiveElement();
-      break;
-  }
+  assert.noUserPrompt(this.dialog);
+
+  resp.body.value = yield this.listener.getActiveElement();
 };
 
 /**
  * Send click event to element.
  *
  * @param {string} id
  *     Reference ID to the element that will be clicked.
+ *
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
  */
-GeckoDriver.prototype.clickElement = function*(cmd, resp) {
-  let win = assert.window(this.getCurrentWindow());
+GeckoDriver.prototype.clickElement = function* (cmd, resp) {
+  const win = assert.window(this.getCurrentWindow());
+  assert.noUserPrompt(this.dialog);
 
   let id = cmd.parameters.id;
 
   switch (this.context) {
     case Context.CHROME:
       let el = this.curBrowser.seenEls.get(id, {frame: win});
       yield interaction.clickElement(el, this.a11yChecks);
       break;
@@ -1907,26 +2036,31 @@ GeckoDriver.prototype.clickElement = fun
  *
  * @param {string} id
  *     Web element reference ID to the element that will be inspected.
  * @param {string} name
  *     Name of the attribute which value to retrieve.
  *
  * @return {string}
  *     Value of the attribute.
+ *
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
  */
-GeckoDriver.prototype.getElementAttribute = function*(cmd, resp) {
-  let win = assert.window(this.getCurrentWindow());
+GeckoDriver.prototype.getElementAttribute = function* (cmd, resp) {
+  const win = assert.window(this.getCurrentWindow());
+  assert.noUserPrompt(this.dialog);
 
   let {id, name} = cmd.parameters;
 
   switch (this.context) {
     case Context.CHROME:
       let el = this.curBrowser.seenEls.get(id, {frame: win});
-
       resp.body.value = el.getAttribute(name);
       break;
 
     case Context.CONTENT:
       resp.body.value = yield this.listener.getElementAttribute(id, name);
       break;
   }
 };
@@ -1936,19 +2070,25 @@ GeckoDriver.prototype.getElementAttribut
  *
  * @param {string} id
  *     Web element reference ID to the element that will be inspected.
  * @param {string} name
  *     Name of the property which value to retrieve.
  *
  * @return {string}
  *     Value of the property.
+ *
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
  */
-GeckoDriver.prototype.getElementProperty = function*(cmd, resp) {
-  let win = assert.window(this.getCurrentWindow());
+GeckoDriver.prototype.getElementProperty = function* (cmd, resp) {
+  const win = assert.window(this.getCurrentWindow());
+  assert.noUserPrompt(this.dialog);
 
   let {id, name} = cmd.parameters;
 
   switch (this.context) {
     case Context.CHROME:
       let el = this.curBrowser.seenEls.get(id, {frame: win});
       resp.body.value = el[name];
       break;
@@ -1960,19 +2100,28 @@ GeckoDriver.prototype.getElementProperty
 };
 
 /**
  * Get the text of an element, if any.  Includes the text of all child
  * elements.
  *
  * @param {string} id
  *     Reference ID to the element that will be inspected.
+ *
+ * @return {string}
+ *     Element's text "as rendered".
+ *
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
  */
-GeckoDriver.prototype.getElementText = function*(cmd, resp) {
-  let win = assert.window(this.getCurrentWindow());
+GeckoDriver.prototype.getElementText = function* (cmd, resp) {
+  const win = assert.window(this.getCurrentWindow());
+  assert.noUserPrompt(this.dialog);
 
   let id = cmd.parameters.id;
 
   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(id, {frame: win});
       let lines = [];
@@ -1986,19 +2135,28 @@ GeckoDriver.prototype.getElementText = f
   }
 };
 
 /**
  * Get the tag name of the element.
  *
  * @param {string} id
  *     Reference ID to the element that will be inspected.
+ *
+ * @return {string}
+ *     Local tag name of element.
+ *
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
  */
-GeckoDriver.prototype.getElementTagName = function*(cmd, resp) {
-  let win = assert.window(this.getCurrentWindow());
+GeckoDriver.prototype.getElementTagName = function* (cmd, resp) {
+  const win = assert.window(this.getCurrentWindow());
+  assert.noUserPrompt(this.dialog);
 
   let id = cmd.parameters.id;
 
   switch (this.context) {
     case Context.CHROME:
       let el = this.curBrowser.seenEls.get(id, {frame: win});
       resp.body.value = el.tagName.toLowerCase();
       break;
@@ -2009,19 +2167,28 @@ GeckoDriver.prototype.getElementTagName 
   }
 };
 
 /**
  * Check if element is displayed.
  *
  * @param {string} id
  *     Reference ID to the element that will be inspected.
+ *
+ * @return {boolean}
+ *     True if displayed, false otherwise.
+ *
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
  */
-GeckoDriver.prototype.isElementDisplayed = function*(cmd, resp) {
-  let win = assert.window(this.getCurrentWindow());
+GeckoDriver.prototype.isElementDisplayed = function* (cmd, resp) {
+  const win = assert.window(this.getCurrentWindow());
+  assert.noUserPrompt(this.dialog);
 
   let id = cmd.parameters.id;
 
   switch (this.context) {
     case Context.CHROME:
       let el = this.curBrowser.seenEls.get(id, {frame: win});
       resp.body.value = yield interaction.isElementDisplayed(
           el, this.a11yChecks);
@@ -2035,19 +2202,28 @@ GeckoDriver.prototype.isElementDisplayed
 
 /**
  * Return the property of the computed style of an element.
  *
  * @param {string} id
  *     Reference ID to the element that will be checked.
  * @param {string} propertyName
  *     CSS rule that is being requested.
+ *
+ * @return {string}
+ *     Value of |propertyName|.
+ *
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
  */
-GeckoDriver.prototype.getElementValueOfCssProperty = function*(cmd, resp) {
-  let win = assert.window(this.getCurrentWindow());
+GeckoDriver.prototype.getElementValueOfCssProperty = function* (cmd, resp) {
+  const win = assert.window(this.getCurrentWindow());
+  assert.noUserPrompt(this.dialog);
 
   let {id, propertyName: prop} = cmd.parameters;
 
   switch (this.context) {
     case Context.CHROME:
       let el = this.curBrowser.seenEls.get(id, {frame: win});
       let sty = win.document.defaultView.getComputedStyle(el);
       resp.body.value = sty.getPropertyValue(prop);
@@ -2059,44 +2235,62 @@ GeckoDriver.prototype.getElementValueOfC
   }
 };
 
 /**
  * Check if element is enabled.
  *
  * @param {string} id
  *     Reference ID to the element that will be checked.
+ *
+ * @return {boolean}
+ *     True if enabled, false if disabled.
+ *
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
  */
-GeckoDriver.prototype.isElementEnabled = function*(cmd, resp) {
-  let win = assert.window(this.getCurrentWindow());
+GeckoDriver.prototype.isElementEnabled = function* (cmd, resp) {
+  const win = assert.window(this.getCurrentWindow());
+  assert.noUserPrompt(this.dialog);
 
   let id = cmd.parameters.id;
 
   switch (this.context) {
     case Context.CHROME:
       // Selenium atom doesn't quite work here
       let el = this.curBrowser.seenEls.get(id, {frame: win});
       resp.body.value = yield interaction.isElementEnabled(
           el, this.a11yChecks);
       break;
 
     case Context.CONTENT:
       resp.body.value = yield this.listener.isElementEnabled(id);
       break;
   }
-},
+};
 
 /**
  * Check if element is selected.
  *
  * @param {string} id
  *     Reference ID to the element that will be checked.
+ *
+ * @return {boolean}
+ *     True if selected, false if unselected.
+ *
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
  */
-GeckoDriver.prototype.isElementSelected = function*(cmd, resp) {
-  let win = assert.window(this.getCurrentWindow());
+GeckoDriver.prototype.isElementSelected = function* (cmd, resp) {
+  const win = assert.window(this.getCurrentWindow());
+  assert.noUserPrompt(this.dialog);
 
   let id = cmd.parameters.id;
 
   switch (this.context) {
     case Context.CHROME:
       // Selenium atom doesn't quite work here
       let el = this.curBrowser.seenEls.get(id, {frame: win});
       resp.body.value = yield interaction.isElementSelected(
@@ -2104,18 +2298,25 @@ GeckoDriver.prototype.isElementSelected 
       break;
 
     case Context.CONTENT:
       resp.body.value = yield this.listener.isElementSelected(id);
       break;
   }
 };
 
-GeckoDriver.prototype.getElementRect = function*(cmd, resp) {
-  let win = assert.window(this.getCurrentWindow());
+/**
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
+ */
+GeckoDriver.prototype.getElementRect = function* (cmd, resp) {
+  const win = assert.window(this.getCurrentWindow());
+  assert.noUserPrompt(this.dialog);
 
   let id = cmd.parameters.id;
 
   switch (this.context) {
     case Context.CHROME:
       let el = this.curBrowser.seenEls.get(id, {frame: win});
       let rect = el.getBoundingClientRect();
       resp.body = {
@@ -2134,19 +2335,26 @@ GeckoDriver.prototype.getElementRect = f
 
 /**
  * Send key presses to element after focusing on it.
  *
  * @param {string} id
  *     Reference ID to the element that will be checked.
  * @param {string} value
  *     Value to send to the element.
+ *
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
  */
-GeckoDriver.prototype.sendKeysToElement = function*(cmd, resp) {
-  let win = assert.window(this.getCurrentWindow());
+GeckoDriver.prototype.sendKeysToElement = function* (cmd, resp) {
+  const win = assert.window(this.getCurrentWindow());
+  assert.noUserPrompt(this.dialog);
+
   let {id, text} = cmd.parameters;
   assert.string(text);
 
   switch (this.context) {
     case Context.CHROME:
       let el = this.curBrowser.seenEls.get(id, {frame: win});
       yield interaction.sendKeysToElement(
           el, text, true, this.a11yChecks);
@@ -2167,19 +2375,25 @@ GeckoDriver.prototype.setTestName = func
   yield this.listener.setTestName({value: val});
 };
 
 /**
  * Clear the text of an element.
  *
  * @param {string} id
  *     Reference ID to the element that will be cleared.
+ *
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
  */
-GeckoDriver.prototype.clearElement = function*(cmd, resp) {
-  let win = assert.window(this.getCurrentWindow());
+GeckoDriver.prototype.clearElement = function* (cmd, resp) {
+  const win = assert.window(this.getCurrentWindow());
+  assert.noUserPrompt(this.dialog);
 
   let id = cmd.parameters.id;
 
   switch (this.context) {
     case Context.CHROME:
       // the selenium atom doesn't work here
       let el = this.curBrowser.seenEls.get(id, {frame: win});
       if (el.nodeName == "textbox") {
@@ -2195,29 +2409,38 @@ GeckoDriver.prototype.clearElement = fun
   }
 };
 
 /**
  * Switch to shadow root of the given host element.
  *
  * @param {string} id element id.
  */
-GeckoDriver.prototype.switchToShadowRoot = function*(cmd, resp) {
-  assert.content(this.context)
+GeckoDriver.prototype.switchToShadowRoot = function* (cmd, resp) {
+  assert.content(this.context);
   assert.window(this.getCurrentWindow());
 
-  let id;
-  if (cmd.parameters) { id = cmd.parameters.id; }
+  let id = cmd.parameters.id;
   yield this.listener.switchToShadowRoot(id);
 };
 
-/** Add a cookie to the document. */
-GeckoDriver.prototype.addCookie = function*(cmd, resp) {
-  assert.content(this.context)
+/**
+ * Add a cookie to the document.
+ *
+ * @throws {UnsupportedOperationError}
+ *     Not available in current context.
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
+ */
+GeckoDriver.prototype.addCookie = function* (cmd, resp) {
+  assert.content(this.context);
   assert.window(this.getCurrentWindow());
+  assert.noUserPrompt(this.dialog);
 
   let cb = msg => {
     this.mm.removeMessageListener("Marionette:addCookie", cb);
     let cookie = msg.json;
     Services.cookies.add(
         cookie.domain,
         cookie.path,
         cookie.name,
@@ -2234,28 +2457,46 @@ GeckoDriver.prototype.addCookie = functi
   yield this.listener.addCookie(cmd.parameters.cookie);
 };
 
 /**
  * Get all the cookies for the current domain.
  *
  * This is the equivalent of calling {@code document.cookie} and parsing
  * the result.
+ *
+ * @throws {UnsupportedOperationError}
+ *     Not available in current context.
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
  */
-GeckoDriver.prototype.getCookies = function*(cmd, resp) {
-  assert.content(this.context)
+GeckoDriver.prototype.getCookies = function* (cmd, resp) {
+  assert.content(this.context);
   assert.window(this.getCurrentWindow());
+  assert.noUserPrompt(this.dialog);
 
   resp.body = yield this.listener.getCookies();
 };
 
-/** Delete all cookies that are visible to a document. */
-GeckoDriver.prototype.deleteAllCookies = function*(cmd, resp) {
-  assert.content(this.context)
+/**
+ * Delete all cookies that are visible to a document.
+ *
+ * @throws {UnsupportedOperationError}
+ *     Not available in current context.
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
+ */
+GeckoDriver.prototype.deleteAllCookies = function* (cmd, resp) {
+  assert.content(this.context);
   assert.window(this.getCurrentWindow());
+  assert.noUserPrompt(this.dialog);
 
   let cb = msg => {
     let cookie = msg.json;
     cookieManager.remove(
         cookie.host,
         cookie.name,
         cookie.path,
         false,
@@ -2263,20 +2504,30 @@ GeckoDriver.prototype.deleteAllCookies =
     return true;
   };
 
   this.mm.addMessageListener("Marionette:deleteCookie", cb);
   yield this.listener.deleteAllCookies();
   this.mm.removeMessageListener("Marionette:deleteCookie", cb);
 };
 
-/** Delete a cookie by name. */
-GeckoDriver.prototype.deleteCookie = function*(cmd, resp) {
-  assert.content(this.context)
+/**
+ * Delete a cookie by name.
+ *
+ * @throws {UnsupportedOperationError}
+ *     Not available in current context.
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
+ */
+GeckoDriver.prototype.deleteCookie = function* (cmd, resp) {
+  assert.content(this.context);
   assert.window(this.getCurrentWindow());
+  assert.noUserPrompt(this.dialog);
 
   let cb = msg => {
     this.mm.removeMessageListener("Marionette:deleteCookie", cb);
     let cookie = msg.json;
     cookieManager.remove(
         cookie.host,
         cookie.name,
         cookie.path,
@@ -2287,26 +2538,33 @@ GeckoDriver.prototype.deleteCookie = fun
 
   this.mm.addMessageListener("Marionette:deleteCookie", cb);
   yield this.listener.deleteCookie(cmd.parameters.name);
 };
 
 /**
  * Close the currently selected tab/window.
  *
- * With multiple open tabs present the currently selected tab will be closed.
- * Otherwise the window itself will be closed. If it is the last window
- * currently open, the window will not be closed to prevent a shutdown of the
- * application. Instead the returned list of window handles is empty.
+ * With multiple open tabs present the currently selected tab will
+ * be closed.  Otherwise the window itself will be closed. If it is the
+ * last window currently open, the window will not be closed to prevent
+ * a shutdown of the application. Instead the returned list of window
+ * handles is empty.
  *
  * @return {Array.<string>}
  *     Unique window handles of remaining windows.
+ *
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
  */
 GeckoDriver.prototype.close = function (cmd, resp) {
   assert.window(this.getCurrentWindow());
+  assert.noUserPrompt(this.dialog);
 
   let nwins = 0;
 
   let winEn = Services.wm.getEnumerator(null);
   while (winEn.hasMoreElements()) {
     let win = winEn.getNext();
 
     // For browser windows count the tabs. Otherwise take the window itself.
@@ -2595,21 +2853,27 @@ GeckoDriver.prototype.setScreenOrientati
     throw new WebDriverError(`Unable to set screen orientation: ${or}`);
   }
 };
 
 /**
  * Maximizes the user agent window as if the user pressed the maximise
  * button.
  *
- * Not Supported on B2G or Fennec.
+ * @throws {UnsupportedOperationError}
+ *     Not available for current application.
+ * @throws {NoSuchWindowError}
+ *     Top-level browsing context has been discarded.
+ * @throws {UnexpectedAlertOpenError}
+ *     A modal dialog is open, blocking this operation.
  */
 GeckoDriver.prototype.maximizeWindow = function (cmd, resp) {
-  assert.firefox()
-  let win = assert.window(this.getCurrentWindow());
+  assert.firefox();
+  const win = assert.window(this.getCurrentWindow());
+  assert.noUserPrompt(this.dialog);
 
   win.maximize()
 };
 
 /**
  * Dismisses a currently displayed tab modal, or returns no such alert if
  * no modal is displayed.
  */

--- a/testing/marionette/error.js
+++ b/testing/marionette/error.js
@@ -21,16 +21,17 @@ const ERRORS = new Set([
   "NoSuchElementError",
   "NoSuchFrameError",
   "NoSuchWindowError",
   "ScriptTimeoutError",
   "SessionNotCreatedError",
   "StaleElementReferenceError",
   "TimeoutError",
   "UnableToSetCookieError",
+  "UnexpectedAlertOpenError",
   "UnknownCommandError",
   "UnknownError",
   "UnsupportedOperationError",
   "WebDriverError",
 ]);
 
 const BUILTIN_ERRORS = new Set([
   "Error",
@@ -445,16 +446,23 @@ class TimeoutError extends WebDriverErro
 
 class UnableToSetCookieError extends WebDriverError {
   constructor (message) {
     super(message);
     this.status = "unable to set cookie";
   }
 }
 
+class UnexpectedAlertOpenError extends WebDriverError {
+  constructor (message) {
+    super(message);
+    this.status = "unexpected alert open";
+  }
+}
+
 class UnknownCommandError extends WebDriverError {
   constructor (message) {
     super(message);
     this.status = "unknown command";
   }
 }
 
 class UnknownError extends WebDriverError {
@@ -467,32 +475,33 @@ class UnknownError extends WebDriverErro
 class UnsupportedOperationError extends WebDriverError {
   constructor (message) {
     super(message);
     this.status = "unsupported operation";
   }
 }
 
 const STATUSES = new Map([
+  ["element click intercepted", ElementClickInterceptedError],
   ["element not accessible", ElementNotAccessibleError],
   ["element not interactable", ElementNotInteractableError],
-  ["element click intercepted", ElementClickInterceptedError],
   ["insecure certificate", InsecureCertificateError],
   ["invalid argument", InvalidArgumentError],
   ["invalid element state", InvalidElementStateError],
   ["invalid selector", InvalidSelectorError],
   ["invalid session id", InvalidSessionIDError],
   ["javascript error", JavaScriptError],
   ["move target out of bounds", MoveTargetOutOfBoundsError],
   ["no alert open", NoAlertOpenError],
   ["no such element", NoSuchElementError],
   ["no such frame", NoSuchFrameError],
   ["no such window", NoSuchWindowError],
   ["script timeout", ScriptTimeoutError],
   ["session not created", SessionNotCreatedError],
   ["stale element reference", StaleElementReferenceError],
   ["timeout", TimeoutError],
   ["unable to set cookie", UnableToSetCookieError],
+  ["unexpected alert open", UnexpectedAlertOpenError],
   ["unknown command", UnknownCommandError],
   ["unknown error", UnknownError],
   ["unsupported operation", UnsupportedOperationError],
   ["webdriver error", WebDriverError],
 ]);

--- a/testing/marionette/harness/marionette_harness/tests/unit/test_modal_dialogs.py
+++ b/testing/marionette/harness/marionette_harness/tests/unit/test_modal_dialogs.py
@@ -1,28 +1,28 @@
 # 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/.
 
 from marionette_driver.by import By
-from marionette_driver.errors import NoAlertPresentException, ElementNotInteractableException
 from marionette_driver.expected import element_present
+from marionette_driver import errors
 from marionette_driver.marionette import Alert
 from marionette_driver.wait import Wait
 
 from marionette_harness import MarionetteTestCase, skip_if_e10s, WindowManagerMixin
 
 
 class BaseAlertTestCase(WindowManagerMixin, MarionetteTestCase):
 
     def alert_present(self):
         try:
             Alert(self.marionette).text
             return True
-        except NoAlertPresentException:
+        except errors.NoAlertPresentException:
             return False
 
     def wait_for_alert(self, timeout=None):
         Wait(self.marionette, timeout=timeout).until(
             lambda _: self.alert_present())
 
     def wait_for_alert_closed(self, timeout=None):
         Wait(self.marionette, timeout=timeout).until(
@@ -48,18 +48,20 @@ class TestTabModalAlerts(BaseAlertTestCa
 
             self.wait_for_alert_closed()
         except:
             pass
 
         super(TestTabModalAlerts, self).tearDown()
 
     def test_no_alert_raises(self):
-        self.assertRaises(NoAlertPresentException, Alert(self.marionette).accept)
-        self.assertRaises(NoAlertPresentException, Alert(self.marionette).dismiss)
+        with self.assertRaises(errors.NoAlertPresentException):
+            Alert(self.marionette).accept()
+        with self.assertRaises(errors.NoAlertPresentException):
+            Alert(self.marionette).dismiss()
 
     def test_alert_accept(self):
         self.marionette.find_element(By.ID, "tab-modal-alert").click()
         self.wait_for_alert()
         alert = self.marionette.switch_to_alert()
         alert.accept()
 
     def test_alert_dismiss(self):
@@ -107,51 +109,53 @@ class TestTabModalAlerts(BaseAlertTestCa
         # Restart the session to ensure we still find the formerly left-open dialog.
         self.marionette.delete_session()
         self.marionette.start_session()
 
         alert = self.marionette.switch_to_alert()
         alert.dismiss()
 
     def test_alert_text(self):
-        with self.assertRaises(NoAlertPresentException):
+        with self.assertRaises(errors.NoAlertPresentException):
             alert = self.marionette.switch_to_alert()
             alert.text
         self.marionette.find_element(By.ID, "tab-modal-alert").click()
         self.wait_for_alert()
         alert = self.marionette.switch_to_alert()
         self.assertEqual(alert.text, "Marionette alert")
         alert.accept()
 
     def test_prompt_text(self):
-        with self.assertRaises(NoAlertPresentException):
+        with self.assertRaises(errors.NoAlertPresentException):
             alert = self.marionette.switch_to_alert()
             alert.text
         self.marionette.find_element(By.ID, "tab-modal-prompt").click()
         self.wait_for_alert()
         alert = self.marionette.switch_to_alert()
         self.assertEqual(alert.text, "Marionette prompt")
         alert.accept()
 
     def test_confirm_text(self):
-        with self.assertRaises(NoAlertPresentException):
+        with self.assertRaises(errors.NoAlertPresentException):
             alert = self.marionette.switch_to_alert()
             alert.text
         self.marionette.find_element(By.ID, "tab-modal-confirm").click()
         self.wait_for_alert()
         alert = self.marionette.switch_to_alert()
         self.assertEqual(alert.text, "Marionette confirm")
         alert.accept()
 
     def test_set_text_throws(self):
-        self.assertRaises(NoAlertPresentException, Alert(self.marionette).send_keys, "Foo")
+        with self.assertRaises(errors.NoAlertPresentException):
+            Alert(self.marionette).send_keys("Foo")
         self.marionette.find_element(By.ID, "tab-modal-alert").click()
         self.wait_for_alert()
         alert = self.marionette.switch_to_alert()
-        self.assertRaises(ElementNotInteractableException, alert.send_keys, "Foo")
+        with self.assertRaises(errors.ElementNotInteractableException):
+            alert.send_keys("Foo")
         alert.accept()
 
     def test_set_text_accept(self):
         self.marionette.find_element(By.ID, "tab-modal-prompt").click()
         self.wait_for_alert()
         alert = self.marionette.switch_to_alert()
         alert.send_keys("Some text!")
         alert.accept()
@@ -189,41 +193,21 @@ class TestTabModalAlerts(BaseAlertTestCa
             """))
         self.marionette.navigate("about:blank")
         self.wait_for_alert()
         alert = self.marionette.switch_to_alert()
         self.assertTrue(alert.text.startswith("This page is asking you to confirm"))
         alert.accept()
         self.wait_for_condition(lambda mn: mn.get_url() == "about:blank")
 
-    @skip_if_e10s("Bug 1325044")
     def test_unrelated_command_when_alert_present(self):
-        click_handler = self.marionette.find_element(By.ID, "click-handler")
-        text = self.marionette.find_element(By.ID, "click-result").text
-        self.assertEqual(text, "")
-
         self.marionette.find_element(By.ID, "tab-modal-alert").click()
         self.wait_for_alert()
-
-        # Commands succeed, but because the dialog blocks the event loop,
-        # our actions aren't reflected on the page.
-        text = self.marionette.find_element(By.ID, "click-result").text
-        self.assertEqual(text, "")
-        click_handler.click()
-        text = self.marionette.find_element(By.ID, "click-result").text
-        self.assertEqual(text, "")
-
-        alert = self.marionette.switch_to_alert()
-        alert.accept()
-
-        self.wait_for_alert_closed()
-
-        click_handler.click()
-        text = self.marionette.find_element(By.ID, "click-result").text
-        self.assertEqual(text, "result")
+        with self.assertRaises(errors.UnexpectedAlertOpen):
+            self.marionette.find_element(By.ID, "click-result")
 
 
 class TestModalAlerts(BaseAlertTestCase):
 
     def setUp(self):
         super(TestModalAlerts, self).setUp()
 
     def tearDown(self):

--- a/testing/marionette/test_assert.js
+++ b/testing/marionette/test_assert.js
@@ -28,16 +28,24 @@ add_test(function test_platforms() {
       raised = e;
     }
   }
   ok(raised instanceof UnsupportedOperationError);
 
   run_next_test();
 });
 
+add_test(function test_noUserPrompt() {
+  assert.noUserPrompt(null);
+  assert.noUserPrompt(undefined);
+  Assert.throws(() => assert.noUserPrompt({}), UnexpectedAlertOpenError);
+
+  run_next_test();
+});
+
 add_test(function test_defined() {
   assert.defined({});
   Assert.throws(() => assert.defined(undefined), InvalidArgumentError);
 
   run_next_test();
 });
 
 add_test(function test_number() {