--- a/browser/components/customizableui/test/browser_967000_button_charEncoding.js
+++ b/browser/components/customizableui/test/browser_967000_button_charEncoding.js
@@ -20,17 +20,17 @@ add_task(function*() {
   ok(charEncodingButton, "The Character Encoding button was added to the Panel Menu");
   is(charEncodingButton.getAttribute("disabled"), "true",
      "The Character encoding button is initially disabled");
 
   let panelHidePromise = promisePanelHidden(window);
   PanelUI.hide();
   yield panelHidePromise;
 
-  let newTab = yield BrowserTestUtils.openNewForegroundTab(gBrowser, TEST_PAGE, true);
+  let newTab = yield BrowserTestUtils.openNewForegroundTab(gBrowser, TEST_PAGE, true, true);
 
   yield PanelUI.show();
   ok(!charEncodingButton.hasAttribute("disabled"), "The Character encoding button gets enabled");
   let characterEncodingView = document.getElementById("PanelUI-characterEncodingView");
   let subviewShownPromise = subviewShown(characterEncodingView);
   charEncodingButton.click();
   yield subviewShownPromise;
 

--- a/browser/components/customizableui/test/browser_987640_charEncoding.js
+++ b/browser/components/customizableui/test/browser_987640_charEncoding.js
@@ -8,17 +8,17 @@ const TEST_PAGE = "http://mochi.test:888
 
 add_task(function*() {
   info("Check Character Encoding panel functionality");
 
   // add the Character Encoding button to the panel
   CustomizableUI.addWidgetToArea("characterencoding-button",
                                   CustomizableUI.AREA_PANEL);
 
-  let newTab = yield BrowserTestUtils.openNewForegroundTab(gBrowser, TEST_PAGE, true);
+  let newTab = yield BrowserTestUtils.openNewForegroundTab(gBrowser, TEST_PAGE, true, true);
 
   yield PanelUI.show();
   let charEncodingButton = document.getElementById("characterencoding-button");
   let characterEncodingView = document.getElementById("PanelUI-characterEncodingView");
   let subviewShownPromise = subviewShown(characterEncodingView);
   charEncodingButton.click();
   yield subviewShownPromise;
 

--- a/testing/mochitest/BrowserTestUtils/BrowserTestUtils.jsm
+++ b/testing/mochitest/BrowserTestUtils/BrowserTestUtils.jsm
@@ -73,38 +73,44 @@ this.BrowserTestUtils = {
    *
    * @param {tabbrowser} tabbrowser
    *        The tabbrowser to open the tab new in.
    * @param {string} opening
    *        May be either a string URL to load in the tab, or a function that
    *        will be called to open a foreground tab. Defaults to "about:blank".
    * @param {boolean} waitForLoad
    *        True to wait for the page in the new tab to load. Defaults to true.
+   * @param {boolean} waitForStateStop
+   *        True to wait for the web progress listener to send STATE_STOP for the
+   *        document in the tab. Defaults to false.
    *
    * @return {Promise}
    *         Resolves when the tab is ready and loaded as necessary.
    * @resolves The new tab.
    */
-  openNewForegroundTab(tabbrowser, opening = "about:blank", aWaitForLoad = true) {
+  openNewForegroundTab(tabbrowser, opening = "about:blank", aWaitForLoad = true, aWaitForStateStop = false) {
     let tab;
     let promises = [
       BrowserTestUtils.switchTab(tabbrowser, function () {
         if (typeof opening == "function") {
           opening();
           tab = tabbrowser.selectedTab;
         }
         else {
           tabbrowser.selectedTab = tab = tabbrowser.addTab(opening);
         }
       })
     ];
 
     if (aWaitForLoad) {
       promises.push(BrowserTestUtils.browserLoaded(tab.linkedBrowser));
     }
+    if (aWaitForStateStop) {
+      promises.push(BrowserTestUtils.browserStopped(tab.linkedBrowser));
+    }
 
     return Promise.all(promises).then(() => tab);
   },
 
   /**
    * Switches to a tab and resolves when it is ready.
    *
    * @param {tabbrowser} tabbrowser
@@ -174,16 +180,52 @@ this.BrowserTestUtils = {
           mm.removeMessageListener("browser-test-utils:loadEvent", onLoad);
           resolve(msg.data.url);
         }
       });
     });
   },
 
   /**
+   * Waits for the web progress listener associated with this tab to fire a
+   * STATE_STOP for the toplevel document.
+   *
+   * @param {xul:browser} browser
+   *        A xul:browser.
+   *
+   * @return {Promise}
+   * @resolves When STATE_STOP reaches the tab's progress listener
+   */
+  browserStopped(browser) {
+    return new Promise(resolve => {
+      let wpl = {
+        onStateChange(aWebProgress, aRequest, aStateFlags, aStatus) {
+          if (aStateFlags & Ci.nsIWebProgressListener.STATE_STOP &&
+              aWebProgress.isTopLevel) {
+            browser.webProgress.removeProgressListener(filter);
+            filter.removeProgressListener(wpl);
+            resolve();
+          };
+        },
+        onSecurityChange() {},
+        onStatusChange() {},
+        onLocationChange() {},
+        QueryInterface: XPCOMUtils.generateQI([
+          Ci.nsIWebProgressListener,
+          Ci.nsIWebProgressListener2,
+        ]),
+      };
+      const filter = Cc["@mozilla.org/appshell/component/browser-status-filter;1"]
+                       .createInstance(Ci.nsIWebProgress);
+      filter.addProgressListener(wpl, Ci.nsIWebProgress.NOTIFY_ALL);
+      browser.webProgress.addProgressListener(filter, Ci.nsIWebProgress.NOTIFY_ALL);
+    });
+  },
+
+  /**
    * Waits for the next tab to open and load a given URL.
    *
    * The method doesn't wait for the tab contents to load.
    *
    * @param {tabbrowser} tabbrowser
    *        The tabbrowser to look for the next new tab in.
    * @param {string} url
    *        A string URL to look for in the new tab. If null, allows any non-blank URL.