--- a/testing/marionette/assert.js
+++ b/testing/marionette/assert.js
@@ -4,27 +4,33 @@
 
 "use strict";
 
 const {utils: Cu} = Components;
 
 Cu.import("resource://gre/modules/AppConstants.jsm");
 Cu.import("resource://gre/modules/Preferences.jsm");
 Cu.import("resource://gre/modules/Services.jsm");
+Cu.import("resource://gre/modules/XPCOMUtils.jsm");
 
 const {
   InvalidArgumentError,
   InvalidSessionIDError,
   JavaScriptError,
   NoSuchWindowError,
   UnexpectedAlertOpenError,
   UnsupportedOperationError,
 } = Cu.import("chrome://marionette/content/error.js", {});
 const {pprint} = Cu.import("chrome://marionette/content/format.js", {});
 
+XPCOMUtils.defineLazyGetter(this, "browser", () => {
+  const {browser} = Cu.import("chrome://marionette/content/browser.js", {});
+  return browser;
+});
+
 this.EXPORTED_SYMBOLS = ["assert"];
 
 const isFennec = () => AppConstants.platform == "android";
 const isFirefox = () =>
     Services.appinfo.ID == "{ec8030f7-c20a-464f-9b0e-13a3a9e97384}";
 
 /**
  * Shorthands for common assertions made in Marionette.
@@ -120,55 +126,46 @@ assert.fennec = function(msg = "") {
  *     If <var>context</var> is not content.
  */
 assert.content = function(context, msg = "") {
   msg = msg || "Only supported in content context";
   assert.that(c => c.toString() == "content", msg, UnsupportedOperationError)(context);
 };
 
 /**
- * Asserts that |win| is open.
- *
- * @param {ChromeWindow} win
- *     Chrome window to test.
- * @param {string=} msg
- *     Custom error message.
- *
- * @return {ChromeWindow}
- *     <var>win</var> is returned unaltered.
+ * Asserts that the {@link ChromeWindow} is open or that the {@link
+ * browser.Context} has a content browser attached.
  *
- * @throws {NoSuchWindowError}
- *     If <var>win</var> has been closed.
- */
-assert.window = function(win, msg = "") {
-  msg = msg || "Unable to locate window";
-  return assert.that(w => w && !w.closed,
-      msg,
-      NoSuchWindowError)(win);
-};
-
-/**
- * Asserts that |context| is a valid browsing context.
+ * When passed in a {@link ChromeContext} this is equivalent to
+ * testing that the associated <code>window</code> global is open,
+ * and when given {@link browser.Context} it will test that the content
+ * frame, represented by <code>&lt;xul:browser&gt;</code>, is
+ * connected.
  *
- * @param {browser.Context} context
+ * @param {(ChromeWindow|browser.Context)} context
  *     Browsing context to test.
  * @param {string=} msg
  *     Custom error message.
  *
+ * @return {(ChromeWindow|browser.Context)}
+ *     <var>context</var> is returned unaltered.
+ *
  * @throws {NoSuchWindowError}
- *     If |context| is invalid.
+ *     If <var>context</var>'s <code>window</code> has been closed.
  */
-assert.contentBrowser = function(context, msg = "") {
+assert.open = function(context, msg = "") {
   // TODO: The contentBrowser uses a cached tab, which is only updated when
   // switchToTab is called. Because of that an additional check is needed to
   // make sure that the chrome window has not already been closed.
-  assert.window(context && context.window);
+  if (context instanceof browser.Context) {
+    assert.open(context.window);
+  }
 
-  msg = msg || "Current window does not have a content browser";
-  assert.that(c => c.contentBrowser,
+  msg = msg || "Browsing context has been discarded";
+  return assert.that(ctx => ctx && !ctx.closed,
       msg,
       NoSuchWindowError)(context);
 };
 
 /**
  * Asserts that there is no current user prompt.
  *
  * @param {modal.Dialog} dialog

--- a/testing/marionette/browser.js
+++ b/testing/marionette/browser.js
@@ -170,16 +170,30 @@ browser.Context = class {
     return null;
   }
 
   get messageManager() {
     return this.contentBrowser.messageManager;
   }
 
   /**
+   * Checks if the browsing context has been discarded.
+   *
+   * The browsing context will have been discarded if the content
+   * browser, represented by the <code>&lt;xul:browser&gt;</code>,
+   * has been detached.
+   *
+   * @return {boolean}
+   *     True if browsing context has been discarded, false otherwise.
+   */
+  get closed() {
+    return this.contentBrowser === null;
+  }
+
+  /**
    * The current frame ID is managed per browser element on desktop in
    * case the ID needs to be refreshed. The currently selected window is
    * identified by a tab.
    */
   get curFrameId() {
     let rv = null;
     if (this.tab || this.driver.isReftestBrowser(this.contentBrowser)) {
       rv = this.getIdForBrowser(this.contentBrowser);

--- a/testing/marionette/driver.js
+++ b/testing/marionette/driver.js
@@ -867,17 +867,17 @@ GeckoDriver.prototype.getContext = funct
  *
  * @throws {ScriptTimeoutError}
  *     If the script was interrupted due to reaching the
  *     <var>scriptTimeout</var> or default timeout.
  * @throws {JavaScriptError}
  *     If an {@link Error} was thrown whilst evaluating the script.
  */
 GeckoDriver.prototype.executeScript = async function(cmd, resp) {
-  assert.window(this.getCurrentWindow());
+  assert.open(this.getCurrentWindow());
 
   let {script, args, scriptTimeout} = cmd.parameters;
   scriptTimeout = scriptTimeout || this.timeouts.script;
 
   let opts = {
     sandboxName: cmd.parameters.sandbox,
     newSandbox: !!(typeof cmd.parameters.newSandbox == "undefined") ||
         cmd.parameters.newSandbox,
@@ -944,17 +944,17 @@ GeckoDriver.prototype.executeScript = as
  *
  * @throws {ScriptTimeoutError}
  *     If the script was interrupted due to reaching the
  *     <var>scriptTimeout</var> or default timeout.
  * @throws {JavaScriptError}
  *     If an Error was thrown whilst evaluating the script.
  */
 GeckoDriver.prototype.executeAsyncScript = async function(cmd, resp) {
-  assert.window(this.getCurrentWindow());
+  assert.open(this.getCurrentWindow());
 
   let {script, args, scriptTimeout} = cmd.parameters;
   scriptTimeout = scriptTimeout || this.timeouts.script;
 
   let opts = {
     sandboxName: cmd.parameters.sandbox,
     newSandbox: !!(typeof cmd.parameters.newSandbox == "undefined") ||
         cmd.parameters.newSandbox,
@@ -1027,17 +1027,17 @@ GeckoDriver.prototype.execute_ = async f
  *     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 = async function(cmd) {
   assert.content(this.context);
-  assert.window(this.getCurrentWindow());
+  assert.open(this.getCurrentWindow());
   this._assertAndDismissModal();
 
   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
@@ -1069,17 +1069,17 @@ GeckoDriver.prototype.get = async functi
  * 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() {
-  assert.window(this.getCurrentWindow());
+  assert.open(this.getCurrentWindow());
   this._assertAndDismissModal();
 
   return this.currentURL.toString();
 };
 
 /**
  * Gets the current title of the window.
  *
@@ -1087,25 +1087,25 @@ GeckoDriver.prototype.getCurrentUrl = fu
  *     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() {
-  assert.window(this.getCurrentWindow());
+  assert.open(this.getCurrentWindow());
   this._assertAndDismissModal();
 
   return this.title;
 };
 
 /** Gets the current type of the window. */
 GeckoDriver.prototype.getWindowType = function(cmd, resp) {
-  assert.window(this.getCurrentWindow());
+  assert.open(this.getCurrentWindow());
 
   resp.body.value = this.windowType;
 };
 
 /**
  * Gets the page source of the content document.
  *
  * @return {string}
@@ -1113,17 +1113,17 @@ GeckoDriver.prototype.getWindowType = fu
  *     active document.
  *
  * @throws {NoSuchWindowError}
  *     Top-level browsing context has been discarded.
  * @throws {UnexpectedAlertOpenError}
  *     A modal dialog is open, blocking this operation.
  */
 GeckoDriver.prototype.getPageSource = async function(cmd, resp) {
-  const win = assert.window(this.getCurrentWindow());
+  const win = assert.open(this.getCurrentWindow());
   this._assertAndDismissModal();
 
   switch (this.context) {
     case Context.Chrome:
       let s = new win.XMLSerializer();
       resp.body.value = s.serializeToString(win.document);
       break;
 
@@ -1141,17 +1141,17 @@ GeckoDriver.prototype.getPageSource = as
  *     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 = async function() {
   assert.content(this.context);
-  assert.contentBrowser(this.curBrowser);
+  assert.open(this.curBrowser);
   this._assertAndDismissModal();
 
   // If there is no history, just return
   if (!this.curBrowser.contentBrowser.webNavigation.canGoBack) {
     return;
   }
 
   let lastURL = this.currentURL;
@@ -1183,17 +1183,17 @@ GeckoDriver.prototype.goBack = async fun
  *     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 = async function() {
   assert.content(this.context);
-  assert.contentBrowser(this.curBrowser);
+  assert.open(this.curBrowser);
   this._assertAndDismissModal();
 
   // If there is no history, just return
   if (!this.curBrowser.contentBrowser.webNavigation.canGoForward) {
     return;
   }
 
   let lastURL = this.currentURL;
@@ -1226,17 +1226,17 @@ GeckoDriver.prototype.goForward = async 
  *     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 = async function() {
   assert.content(this.context);
-  assert.window(this.getCurrentWindow());
+  assert.open(this.getCurrentWindow());
   this._assertAndDismissModal();
 
   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.
@@ -1293,17 +1293,17 @@ GeckoDriver.prototype.getIdForBrowser = 
  *
  * @return {string}
  *     Unique window handle.
  *
  * @throws {NoSuchWindowError}
  *     Top-level browsing context has been discarded.
  */
 GeckoDriver.prototype.getWindowHandle = function() {
-  assert.contentBrowser(this.curBrowser);
+  assert.open(this.curBrowser);
 
   return this.curBrowser.curFrameId.toString();
 };
 
 /**
  * Get a list of top-level browsing contexts. On desktop this typically
  * corresponds to the set of open tabs for browser windows, or the window
  * itself for non-browser chrome windows.
@@ -1328,17 +1328,17 @@ GeckoDriver.prototype.getWindowHandles =
  *
  * @return {string}
  *     Unique window handle.
  *
  * @throws {NoSuchWindowError}
  *     Top-level browsing context has been discarded.
  */
 GeckoDriver.prototype.getChromeWindowHandle = function(cmd, resp) {
-  assert.window(this.getCurrentWindow(Context.Chrome));
+  assert.open(this.getCurrentWindow(Context.Chrome));
 
   for (let i in this.browsers) {
     if (this.curBrowser == this.browsers[i]) {
       resp.body.value = i;
       return;
     }
   }
 };
@@ -1366,17 +1366,17 @@ GeckoDriver.prototype.getChromeWindowHan
  *     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() {
-  assert.window(this.getCurrentWindow());
+  assert.open(this.getCurrentWindow());
   this._assertAndDismissModal();
   return this.curBrowser.rect;
 };
 
 /**
  * Set the window position and size of the browser on the operating
  * system window manager.
  *
@@ -1403,17 +1403,17 @@ GeckoDriver.prototype.getWindowRect = fu
  *     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 = async function(cmd) {
   assert.firefox();
-  const win = assert.window(this.getCurrentWindow());
+  const win = assert.open(this.getCurrentWindow());
   this._assertAndDismissModal();
 
   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 => {
@@ -1602,17 +1602,17 @@ GeckoDriver.prototype.setWindowHandle = 
     if ("tabIndex" in winProperties) {
       this.curBrowser.switchToTab(
           winProperties.tabIndex, winProperties.win, focus);
     }
   }
 };
 
 GeckoDriver.prototype.getActiveFrame = function(cmd, resp) {
-  assert.window(this.getCurrentWindow());
+  assert.open(this.getCurrentWindow());
 
   switch (this.context) {
     case Context.Chrome:
       // no frame means top-level
       resp.body.value = null;
       if (this.curFrame) {
         resp.body.value = this.curBrowser.seenEls.add(
             this.curFrame.frameElement);
@@ -1633,17 +1633,17 @@ GeckoDriver.prototype.getActiveFrame = f
  * 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 = async function() {
-  assert.window(this.getCurrentWindow());
+  assert.open(this.getCurrentWindow());
   this._assertAndDismissModal();
 
   await this.listener.switchToParentFrame();
 };
 
 /**
  * Switch to a given frame within the current window.
  *
@@ -1654,17 +1654,17 @@ GeckoDriver.prototype.switchToParentFram
  *     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 = async function(cmd) {
-  assert.window(this.getCurrentWindow());
+  assert.open(this.getCurrentWindow());
   this._assertAndDismissModal();
 
   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") {
@@ -1837,17 +1837,17 @@ GeckoDriver.prototype.getTimeouts = func
 GeckoDriver.prototype.setTimeouts = function(cmd) {
   // merge with existing timeouts
   let merged = Object.assign(this.timeouts.toJSON(), cmd.parameters);
   this.timeouts = session.Timeouts.fromJSON(merged);
 };
 
 /** Single tap. */
 GeckoDriver.prototype.singleTap = async function(cmd) {
-  assert.window(this.getCurrentWindow());
+  assert.open(this.getCurrentWindow());
 
   let {id, x, y} = cmd.parameters;
   let webEl = WebElement.fromUUID(id, this.context);
 
   switch (this.context) {
     case Context.Chrome:
       throw new UnsupportedOperationError(
           "Command 'singleTap' is not yet available in chrome context");
@@ -1869,17 +1869,17 @@ GeckoDriver.prototype.singleTap = async 
  * @throws {NoSuchWindowError}
  *     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.window(this.getCurrentWindow());
+  assert.open(this.getCurrentWindow());
   this._assertAndDismissModal();
 
   let actions = cmd.parameters.actions;
   await this.listener.performActions({"actions": actions});
 };
 
 /**
  * Release all the keys and pointer buttons that are currently depressed.
@@ -1888,17 +1888,17 @@ GeckoDriver.prototype.performActions = a
  *     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 = async function() {
   assert.content(this.context);
-  assert.window(this.getCurrentWindow());
+  assert.open(this.getCurrentWindow());
   this._assertAndDismissModal();
 
   await this.listener.releaseActions();
 };
 
 /**
  * An action chain.
  *
@@ -1912,17 +1912,17 @@ GeckoDriver.prototype.releaseActions = a
  * @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 = async function(cmd, resp) {
-  const win = assert.window(this.getCurrentWindow());
+  const win = assert.open(this.getCurrentWindow());
   this._assertAndDismissModal();
 
   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
@@ -1950,17 +1950,17 @@ GeckoDriver.prototype.actionChain = asyn
  *     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 = async function(cmd) {
   assert.content(this.context);
-  assert.window(this.getCurrentWindow());
+  assert.open(this.getCurrentWindow());
   this._assertAndDismissModal();
 
   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.
@@ -1971,17 +1971,17 @@ GeckoDriver.prototype.multiAction = asyn
  *     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 = async function(cmd, resp) {
-  const win = assert.window(this.getCurrentWindow());
+  const win = assert.open(this.getCurrentWindow());
   this._assertAndDismissModal();
 
   let {using, value} = cmd.parameters;
   let startNode;
   if (typeof cmd.parameters.element != "undefined") {
     startNode = WebElement.fromUUID(cmd.parameters.element, this.context);
   }
 
@@ -2017,17 +2017,17 @@ GeckoDriver.prototype.findElement = asyn
  * Find elements using the indicated search strategy.
  *
  * @param {string} using
  *     Indicates which search method to use.
  * @param {string} value
  *     Value the client is looking for.
  */
 GeckoDriver.prototype.findElements = async function(cmd, resp) {
-  const win = assert.window(this.getCurrentWindow());
+  const win = assert.open(this.getCurrentWindow());
 
   let {using, value} = cmd.parameters;
   let startNode;
   if (typeof cmd.parameters.element != "undefined") {
     startNode = WebElement.fromUUID(cmd.parameters.element, this.context);
   }
 
   let opts = {
@@ -2071,17 +2071,17 @@ GeckoDriver.prototype.findElements = asy
  * @throws {UnexpectedAlertOpenError}
  *     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.window(this.getCurrentWindow());
+  assert.open(this.getCurrentWindow());
   this._assertAndDismissModal();
 
   return this.listener.getActiveElement();
 };
 
 /**
  * Send click event to element.
  *
@@ -2093,17 +2093,17 @@ GeckoDriver.prototype.getActiveElement =
  * @throws {NoSuchElementError}
  *     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.window(this.getCurrentWindow());
+  assert.open(this.getCurrentWindow());
   this._assertAndDismissModal();
 
   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);
@@ -2149,17 +2149,17 @@ GeckoDriver.prototype.clickElement = asy
  * @throws {NoSuchElementError}
  *     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, resp) {
-  assert.window(this.getCurrentWindow());
+  assert.open(this.getCurrentWindow());
   this._assertAndDismissModal();
 
   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:
@@ -2189,17 +2189,17 @@ GeckoDriver.prototype.getElementAttribut
  * @throws {NoSuchElementError}
  *     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, resp) {
-  assert.window(this.getCurrentWindow());
+  assert.open(this.getCurrentWindow());
   this._assertAndDismissModal();
 
   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:
@@ -2228,17 +2228,17 @@ GeckoDriver.prototype.getElementProperty
  * @throws {NoSuchElementError}
  *     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, resp) {
-  assert.window(this.getCurrentWindow());
+  assert.open(this.getCurrentWindow());
   this._assertAndDismissModal();
 
   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
@@ -2268,17 +2268,17 @@ GeckoDriver.prototype.getElementText = a
  * @throws {NoSuchElementError}
  *     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, resp) {
-  assert.window(this.getCurrentWindow());
+  assert.open(this.getCurrentWindow());
   this._assertAndDismissModal();
 
   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);
@@ -2305,17 +2305,17 @@ GeckoDriver.prototype.getElementTagName 
  * @throws {NoSuchElementError}
  *     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, resp) {
-  assert.window(this.getCurrentWindow());
+  assert.open(this.getCurrentWindow());
   this._assertAndDismissModal();
 
   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);
@@ -2346,17 +2346,17 @@ 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, resp) {
-  const win = assert.window(this.getCurrentWindow());
+  const win = assert.open(this.getCurrentWindow());
   this._assertAndDismissModal();
 
   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:
@@ -2386,17 +2386,17 @@ GeckoDriver.prototype.getElementValueOfC
  * @throws {NoSuchElementError}
  *     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, resp) {
-  assert.window(this.getCurrentWindow());
+  assert.open(this.getCurrentWindow());
   this._assertAndDismissModal();
 
   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
@@ -2425,17 +2425,17 @@ GeckoDriver.prototype.isElementEnabled =
  * @throws {NoSuchElementError}
  *     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, resp) {
-  assert.window(this.getCurrentWindow());
+  assert.open(this.getCurrentWindow());
   this._assertAndDismissModal();
 
   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
@@ -2456,17 +2456,17 @@ GeckoDriver.prototype.isElementSelected 
  * @throws {NoSuchElementError}
  *     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, resp) {
-  const win = assert.window(this.getCurrentWindow());
+  const win = assert.open(this.getCurrentWindow());
   this._assertAndDismissModal();
 
   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);
@@ -2498,17 +2498,17 @@ GeckoDriver.prototype.getElementRect = a
  * @throws {NoSuchElementError}
  *     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.window(this.getCurrentWindow());
+  assert.open(this.getCurrentWindow());
   this._assertAndDismissModal();
 
   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:
@@ -2533,17 +2533,17 @@ GeckoDriver.prototype.sendKeysToElement 
  * @throws {NoSuchElementError}
  *     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.window(this.getCurrentWindow());
+  assert.open(this.getCurrentWindow());
   this._assertAndDismissModal();
 
   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
@@ -2569,17 +2569,17 @@ GeckoDriver.prototype.clearElement = asy
  *
  * @throws {InvalidArgumentError}
  *     If <var>id</var> is not a string.
  * @throws {NoSuchElementError}
  *     If element represented by reference <var>id</var> is unknown.
  */
 GeckoDriver.prototype.switchToShadowRoot = async function(cmd) {
   assert.content(this.context);
-  assert.window(this.getCurrentWindow());
+  assert.open(this.getCurrentWindow());
 
   let id = assert.string(cmd.parameters.id);
   let webEl = WebElement.fromUUID(id, this.context);
   await this.listener.switchToShadowRoot(webEl);
 };
 
 /**
  * Add a single cookie to the cookie store associated with the active
@@ -2595,17 +2595,17 @@ GeckoDriver.prototype.switchToShadowRoot
  * @throws {UnexpectedAlertOpenError}
  *     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.window(this.getCurrentWindow());
+  assert.open(this.getCurrentWindow());
   this._assertAndDismissModal();
 
   let {protocol, hostname} = this.currentURL;
 
   const networkSchemes = ["ftp:", "http:", "https:"];
   if (!networkSchemes.includes(protocol)) {
     throw new InvalidCookieDomainError("Document is cookie-averse");
   }
@@ -2625,17 +2625,17 @@ GeckoDriver.prototype.addCookie = functi
  *     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);
-  assert.window(this.getCurrentWindow());
+  assert.open(this.getCurrentWindow());
   this._assertAndDismissModal();
 
   let {hostname, pathname} = this.currentURL;
   resp.body = [...cookie.iter(hostname, pathname)];
 };
 
 /**
  * Delete all cookies that are visible to a document.
@@ -2644,17 +2644,17 @@ GeckoDriver.prototype.getCookies = funct
  *     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() {
   assert.content(this.context);
-  assert.window(this.getCurrentWindow());
+  assert.open(this.getCurrentWindow());
   this._assertAndDismissModal();
 
   let {hostname, pathname} = this.currentURL;
   for (let toDelete of cookie.iter(hostname, pathname)) {
     cookie.remove(toDelete);
   }
 };
 
@@ -2665,17 +2665,17 @@ GeckoDriver.prototype.deleteAllCookies =
  *     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) {
   assert.content(this.context);
-  assert.window(this.getCurrentWindow());
+  assert.open(this.getCurrentWindow());
   this._assertAndDismissModal();
 
   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);
     }
@@ -2695,17 +2695,17 @@ GeckoDriver.prototype.deleteCookie = fun
  *     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 = async function() {
-  assert.window(this.getCurrentWindow(Context.Content));
+  assert.open(this.getCurrentWindow(Context.Content));
   this._assertAndDismissModal();
 
   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) {
@@ -2733,17 +2733,17 @@ GeckoDriver.prototype.close = async func
  * closed to prevent a shutdown of the application. Instead the returned
  * list of chrome window handles is empty.
  *
  * @return {Array.<string>}
  *     Unique chrome window handles of remaining chrome windows.
  */
 GeckoDriver.prototype.closeChromeWindow = async function() {
   assert.firefox();
-  assert.window(this.getCurrentWindow(Context.Chrome));
+  assert.open(this.getCurrentWindow(Context.Chrome));
 
   let nwins = 0;
 
   // eslint-disable-next-line
   for (let _ of this.windows) {
     nwins++;
   }
 
@@ -2840,17 +2840,17 @@ GeckoDriver.prototype.deleteSession = fu
  *     scroll to the element.
  *
  * @return {string}
  *     If <var>hash</var> is false, PNG image encoded as Base64 encoded
  *     string.  If <var>hash</var> is true, hex digest of the SHA-256
  *     hash of the Base64 encoded string.
  */
 GeckoDriver.prototype.takeScreenshot = function(cmd) {
-  let win = assert.window(this.getCurrentWindow());
+  let win = assert.open(this.getCurrentWindow());
 
   let {id, highlights, full, hash} = cmd.parameters;
   highlights = highlights || [];
   let format = hash ? capture.Format.Hash : capture.Format.Base64;
 
   switch (this.context) {
     case Context.Chrome:
       let highlightEls = highlights
@@ -2895,17 +2895,17 @@ GeckoDriver.prototype.takeScreenshot = f
  * Get the current browser orientation.
  *
  * Will return one of the valid primary orientation values
  * portrait-primary, landscape-primary, portrait-secondary, or
  * landscape-secondary.
  */
 GeckoDriver.prototype.getScreenOrientation = function(cmd, resp) {
   assert.fennec();
-  let win = assert.window(this.getCurrentWindow());
+  let win = assert.open(this.getCurrentWindow());
 
   resp.body.value = win.screen.mozOrientation;
 };
 
 /**
  * Set the current browser orientation.
  *
  * The supplied orientation should be given as one of the valid
@@ -2913,17 +2913,17 @@ GeckoDriver.prototype.getScreenOrientati
  * be raised.
  *
  * Valid orientations are "portrait" and "landscape", which fall
  * back to "portrait-primary" and "landscape-primary" respectively,
  * and "portrait-secondary" as well as "landscape-secondary".
  */
 GeckoDriver.prototype.setScreenOrientation = function(cmd) {
   assert.fennec();
-  let win = assert.window(this.getCurrentWindow());
+  let win = assert.open(this.getCurrentWindow());
 
   const ors = [
     "portrait", "landscape",
     "portrait-primary", "landscape-primary",
     "portrait-secondary", "landscape-secondary",
   ];
 
   let or = String(cmd.parameters.orientation);
@@ -2953,17 +2953,17 @@ GeckoDriver.prototype.setScreenOrientati
  *     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.minimizeWindow = async function() {
   assert.firefox();
-  const win = assert.window(this.getCurrentWindow());
+  const win = assert.open(this.getCurrentWindow());
   this._assertAndDismissModal();
 
   if (WindowState.from(win.windowState) == WindowState.Fullscreen) {
     await exitFullscreen(win);
   }
 
   if (WindowState.from(win.windowState) != WindowState.Minimized) {
     await new Promise(resolve => {
@@ -2990,17 +2990,17 @@ GeckoDriver.prototype.minimizeWindow = a
  *     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 = async function() {
   assert.firefox();
-  const win = assert.window(this.getCurrentWindow());
+  const win = assert.open(this.getCurrentWindow());
   this._assertAndDismissModal();
 
   switch (WindowState.from(win.windowState)) {
     case WindowState.Fullscreen:
       await exitFullscreen(win);
       break;
 
     case WindowState.Minimized:
@@ -3076,17 +3076,17 @@ GeckoDriver.prototype.maximizeWindow = a
  *     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.fullscreenWindow = async function() {
   assert.firefox();
-  const win = assert.window(this.getCurrentWindow());
+  const win = assert.open(this.getCurrentWindow());
   this._assertAndDismissModal();
 
   if (WindowState.from(win.windowState) == WindowState.Minimized) {
     await restoreWindow(win, this.curBrowser.eventObserver);
   }
 
   if (WindowState.from(win.windowState) != WindowState.Fullscreen) {
     await new Promise(resolve => {
@@ -3098,43 +3098,43 @@ GeckoDriver.prototype.fullscreenWindow =
   return this.curBrowser.rect;
 };
 
 /**
  * Dismisses a currently displayed tab modal, or returns no such alert if
  * no modal is displayed.
  */
 GeckoDriver.prototype.dismissDialog = function() {
-  assert.window(this.getCurrentWindow());
+  assert.open(this.getCurrentWindow());
   this._checkIfAlertIsPresent();
 
   let {button0, button1} = this.dialog.ui;
   (button1 ? button1 : button0).click();
   this.dialog = null;
 };
 
 /**
  * Accepts a currently displayed tab modal, or returns no such alert if
  * no modal is displayed.
  */
 GeckoDriver.prototype.acceptDialog = function() {
-  assert.window(this.getCurrentWindow());
+  assert.open(this.getCurrentWindow());
   this._checkIfAlertIsPresent();
 
   let {button0} = this.dialog.ui;
   button0.click();
   this.dialog = null;
 };
 
 /**
  * Returns the message shown in a currently displayed modal, or returns
  * a no such alert error if no modal is currently displayed.
  */
 GeckoDriver.prototype.getTextFromDialog = function(cmd, resp) {
-  assert.window(this.getCurrentWindow());
+  assert.open(this.getCurrentWindow());
   this._checkIfAlertIsPresent();
 
   let {infoBody} = this.dialog.ui;
   resp.body.value = infoBody.textContent;
 };
 
 /**
  * Set the user prompt's value field.
@@ -3151,17 +3151,17 @@ GeckoDriver.prototype.getTextFromDialog 
  *     If the current user prompt is an alert or confirm.
  * @throws {NoSuchAlertError}
  *     If there is no current user prompt.
  * @throws {UnsupportedOperationError}
  *     If the current user prompt is something other than an alert,
  *     confirm, or a prompt.
  */
 GeckoDriver.prototype.sendKeysToDialog = async function(cmd) {
-  assert.window(this.getCurrentWindow());
+  assert.open(this.getCurrentWindow());
   this._checkIfAlertIsPresent();
 
   // see toolkit/components/prompts/content/commonDialog.js
   let {loginTextbox} = this.dialog.ui;
   await interaction.sendKeysToElement(
       loginTextbox, cmd.parameters.text, this.a11yChecks);
 };
 

--- a/testing/marionette/test_assert.js
+++ b/testing/marionette/test_assert.js
@@ -164,37 +164,24 @@ add_test(function test_string() {
   assert.string(`bar`);
   Assert.throws(() => assert.string(42), InvalidArgumentError);
 
   Assert.throws(() => assert.string(42, "custom"), /custom/);
 
   run_next_test();
 });
 
-add_test(function test_window() {
-  assert.window({closed: false});
+add_test(function test_open() {
+  assert.open({closed: false});
 
   for (let typ of [null, undefined, {closed: true}]) {
-    Assert.throws(() => assert.window(typ), NoSuchWindowError);
+    Assert.throws(() => assert.open(typ), NoSuchWindowError);
   }
 
-  Assert.throws(() => assert.window(null, "custom"), /custom/);
-
-  run_next_test();
-});
-
-add_test(function test_contentBrowser() {
-  assert.contentBrowser({contentBrowser: 42, window: {closed: false}});
-
-  let closedWindow = {contentBrowser: 42, window: {closed: true}};
-  let noContentBrowser = {contentBrowser: null, window: {closed: false}};
-
-  for (let typ of [null, undefined, closedWindow, noContentBrowser]) {
-    Assert.throws(() => assert.contentBrowser(typ), NoSuchWindowError);
-  }
+  Assert.throws(() => assert.open(null, "custom"), /custom/);
 
   run_next_test();
 });
 
 add_test(function test_object() {
   assert.object({});
   assert.object(new Object());
   for (let typ of [42, "foo", true, null, undefined]) {