--- a/browser/components/customizableui/content/panelUI.js
+++ b/browser/components/customizableui/content/panelUI.js
@@ -1,18 +1,16 @@
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this file,
  * You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 XPCOMUtils.defineLazyModuleGetter(this, "CustomizableUI",
                                   "resource:///modules/CustomizableUI.jsm");
 XPCOMUtils.defineLazyModuleGetter(this, "ScrollbarSampler",
                                   "resource:///modules/ScrollbarSampler.jsm");
-XPCOMUtils.defineLazyModuleGetter(this, "Promise",
-                                  "resource://gre/modules/Promise.jsm");
 XPCOMUtils.defineLazyModuleGetter(this, "ShortcutUtils",
                                   "resource://gre/modules/ShortcutUtils.jsm");
 XPCOMUtils.defineLazyModuleGetter(this, "AppConstants",
                                   "resource://gre/modules/AppConstants.jsm");
 
 /**
  * Maintains the state and dispatches events for the main menu panel.
  */
@@ -122,58 +120,56 @@ const PanelUI = {
   /**
    * Opens the menu panel. If the event target has a child with the
    * toolbarbutton-icon attribute, the panel will be anchored on that child.
    * Otherwise, the panel is anchored on the event target itself.
    *
    * @param aEvent the event (if any) that triggers showing the menu.
    */
   show: function(aEvent) {
-    let deferred = Promise.defer();
-
-    this.ensureReady().then(() => {
-      if (this.panel.state == "open" ||
-          document.documentElement.hasAttribute("customizing")) {
-        deferred.resolve();
-        return;
-      }
+    return new Promise(resolve => {
+      this.ensureReady().then(() => {
+        if (this.panel.state == "open" ||
+            document.documentElement.hasAttribute("customizing")) {
+          resolve();
+          return;
+        }
 
-      let editControlPlacement = CustomizableUI.getPlacementOfWidget("edit-controls");
-      if (editControlPlacement && editControlPlacement.area == CustomizableUI.AREA_PANEL) {
-        updateEditUIVisibility();
-      }
+        let editControlPlacement = CustomizableUI.getPlacementOfWidget("edit-controls");
+        if (editControlPlacement && editControlPlacement.area == CustomizableUI.AREA_PANEL) {
+          updateEditUIVisibility();
+        }
 
-      let personalBookmarksPlacement = CustomizableUI.getPlacementOfWidget("personal-bookmarks");
-      if (personalBookmarksPlacement &&
-          personalBookmarksPlacement.area == CustomizableUI.AREA_PANEL) {
-        PlacesToolbarHelper.customizeChange();
-      }
+        let personalBookmarksPlacement = CustomizableUI.getPlacementOfWidget("personal-bookmarks");
+        if (personalBookmarksPlacement &&
+            personalBookmarksPlacement.area == CustomizableUI.AREA_PANEL) {
+          PlacesToolbarHelper.customizeChange();
+        }
 
-      let anchor;
-      if (!aEvent ||
-          aEvent.type == "command") {
-        anchor = this.menuButton;
-      } else {
-        anchor = aEvent.target;
-      }
+        let anchor;
+        if (!aEvent ||
+            aEvent.type == "command") {
+          anchor = this.menuButton;
+        } else {
+          anchor = aEvent.target;
+        }
 
-      this.panel.addEventListener("popupshown", function onPopupShown() {
-        this.removeEventListener("popupshown", onPopupShown);
-        deferred.resolve();
-      });
+        this.panel.addEventListener("popupshown", function onPopupShown() {
+          this.removeEventListener("popupshown", onPopupShown);
+          resolve();
+        });
 
-      let iconAnchor =
-        document.getAnonymousElementByAttribute(anchor, "class",
-                                                "toolbarbutton-icon");
-      this.panel.openPopup(iconAnchor || anchor);
-    }, (reason) => {
-      console.error("Error showing the PanelUI menu", reason);
+        let iconAnchor =
+          document.getAnonymousElementByAttribute(anchor, "class",
+                                                  "toolbarbutton-icon");
+        this.panel.openPopup(iconAnchor || anchor);
+      }, (reason) => {
+        console.error("Error showing the PanelUI menu", reason);
+      });
     });
-
-    return deferred.promise;
   },
 
   /**
    * If the menu panel is being shown, hide it.
    */
   hide: function() {
     if (document.documentElement.hasAttribute("customizing")) {
       return;
@@ -226,25 +222,25 @@ const PanelUI = {
    * @return a Promise that resolves once the panel is ready to roll.
    */
   ensureReady: function(aCustomizing=false) {
     if (this._readyPromise) {
       return this._readyPromise;
     }
     this._readyPromise = Task.spawn(function*() {
       if (!this._initialized) {
-        let delayedStartupDeferred = Promise.defer();
-        let delayedStartupObserver = (aSubject, aTopic, aData) => {
-          if (aSubject == window) {
-            Services.obs.removeObserver(delayedStartupObserver, "browser-delayed-startup-finished");
-            delayedStartupDeferred.resolve();
-          }
-        };
-        Services.obs.addObserver(delayedStartupObserver, "browser-delayed-startup-finished", false);
-        yield delayedStartupDeferred.promise;
+        yield new Promise(resolve => {
+          let delayedStartupObserver = (aSubject, aTopic, aData) => {
+            if (aSubject == window) {
+              Services.obs.removeObserver(delayedStartupObserver, "browser-delayed-startup-finished");
+              resolve();
+            }
+          };
+          Services.obs.addObserver(delayedStartupObserver, "browser-delayed-startup-finished", false);
+        });
       }
 
       this.contents.setAttributeNS("http://www.w3.org/XML/1998/namespace", "lang",
                                    getLocale());
       if (!this._scrollWidth) {
         // In order to properly center the contents of the panel, while ensuring
         // that we have enough space on either side to show a scrollbar, we have to
         // do a bit of hackery. In particular, we calculate a new width for the