--- a/browser/components/customizableui/CustomizableUI.jsm
+++ b/browser/components/customizableui/CustomizableUI.jsm
@@ -14,18 +14,16 @@ Cu.import("resource://gre/modules/AppCon
 XPCOMUtils.defineLazyModuleGetter(this, "PanelWideWidgetTracker",
   "resource:///modules/PanelWideWidgetTracker.jsm");
 XPCOMUtils.defineLazyModuleGetter(this, "CustomizableWidgets",
   "resource:///modules/CustomizableWidgets.jsm");
 XPCOMUtils.defineLazyModuleGetter(this, "DeferredTask",
   "resource://gre/modules/DeferredTask.jsm");
 XPCOMUtils.defineLazyModuleGetter(this, "PrivateBrowsingUtils",
   "resource://gre/modules/PrivateBrowsingUtils.jsm");
-XPCOMUtils.defineLazyModuleGetter(this, "Promise",
-  "resource://gre/modules/Promise.jsm");
 XPCOMUtils.defineLazyGetter(this, "gWidgetsBundle", function() {
   const kUrl = "chrome://browser/locale/customizableui/customizableWidgets.properties";
   return Services.strings.createBundle(kUrl);
 });
 XPCOMUtils.defineLazyModuleGetter(this, "ShortcutUtils",
   "resource://gre/modules/ShortcutUtils.jsm");
 XPCOMUtils.defineLazyServiceGetter(this, "gELS",
   "@mozilla.org/eventlistenerservice;1", "nsIEventListenerService");
@@ -4104,38 +4102,36 @@ OverflowableToolbar.prototype = {
         this._onPanelHiding(aEvent);
         break;
       case "resize":
         this._onResize(aEvent);
     }
   },
 
   show: function() {
-    let deferred = Promise.defer();
     if (this._panel.state == "open") {
-      deferred.resolve();
-      return deferred.promise;
-    }
-    let doc = this._panel.ownerDocument;
-    this._panel.hidden = false;
-    let contextMenu = doc.getElementById(this._panel.getAttribute("context"));
-    gELS.addSystemEventListener(contextMenu, 'command', this, true);
-    let anchor = doc.getAnonymousElementByAttribute(this._chevron, "class", "toolbarbutton-icon");
-    this._panel.openPopup(anchor || this._chevron);
-    this._chevron.open = true;
-
-    let overflowableToolbarInstance = this;
-    this._panel.addEventListener("popupshown", function onPopupShown(aEvent) {
-      this.removeEventListener("popupshown", onPopupShown);
-      this.addEventListener("dragover", overflowableToolbarInstance);
-      this.addEventListener("dragend", overflowableToolbarInstance);
-      deferred.resolve();
+      return Promise.resolve();
+    }
+    return new Promise(resolve => {
+      let doc = this._panel.ownerDocument;
+      this._panel.hidden = false;
+      let contextMenu = doc.getElementById(this._panel.getAttribute("context"));
+      gELS.addSystemEventListener(contextMenu, 'command', this, true);
+      let anchor = doc.getAnonymousElementByAttribute(this._chevron, "class", "toolbarbutton-icon");
+      this._panel.openPopup(anchor || this._chevron);
+      this._chevron.open = true;
+
+      let overflowableToolbarInstance = this;
+      this._panel.addEventListener("popupshown", function onPopupShown(aEvent) {
+        this.removeEventListener("popupshown", onPopupShown);
+        this.addEventListener("dragover", overflowableToolbarInstance);
+        this.addEventListener("dragend", overflowableToolbarInstance);
+        resolve();
+      });
     });
-
-    return deferred.promise;
   },
 
   _onClickChevron: function(aEvent) {
     if (this._chevron.open) {
       this._panel.hidePopup();
       this._chevron.open = false;
     } else {
       this.show();

--- a/browser/components/customizableui/ScrollbarSampler.jsm
+++ b/browser/components/customizableui/ScrollbarSampler.jsm
@@ -5,66 +5,61 @@
 "use strict";
 
 this.EXPORTED_SYMBOLS = ["ScrollbarSampler"];
 
 const {classes: Cc, interfaces: Ci, utils: Cu, results: Cr} = Components;
 
 Cu.import("resource://gre/modules/Services.jsm");
 Cu.import("resource://gre/modules/XPCOMUtils.jsm");
-XPCOMUtils.defineLazyModuleGetter(this, "Promise",
-                                  "resource://gre/modules/Promise.jsm");
 
 var gSystemScrollbarWidth = null;
 
 this.ScrollbarSampler = {
   getSystemScrollbarWidth: function() {
-    let deferred = Promise.defer();
-
     if (gSystemScrollbarWidth !== null) {
-      deferred.resolve(gSystemScrollbarWidth);
-      return deferred.promise;
+      return Promise.resolve(gSystemScrollbarWidth);
     }
 
-    this._sampleSystemScrollbarWidth().then(function(systemScrollbarWidth) {
-      gSystemScrollbarWidth = systemScrollbarWidth;
-      deferred.resolve(gSystemScrollbarWidth);
+    return new Promise(resolve => {
+      this._sampleSystemScrollbarWidth().then(function(systemScrollbarWidth) {
+        gSystemScrollbarWidth = systemScrollbarWidth;
+        resolve(gSystemScrollbarWidth);
+      });
     });
-    return deferred.promise;
   },
 
   resetSystemScrollbarWidth: function() {
     gSystemScrollbarWidth = null;
   },
 
   _sampleSystemScrollbarWidth: function() {
-    let deferred = Promise.defer();
     let hwin = Services.appShell.hiddenDOMWindow;
     let hdoc = hwin.document.documentElement;
     let iframe = hwin.document.createElementNS("http://www.w3.org/1999/xhtml",
                                                "html:iframe");
     iframe.setAttribute("srcdoc", '<body style="overflow-y: scroll"></body>');
     hdoc.appendChild(iframe);
 
     let cwindow = iframe.contentWindow;
     let utils = cwindow.QueryInterface(Ci.nsIInterfaceRequestor)
                        .getInterface(Ci.nsIDOMWindowUtils);
 
-    cwindow.addEventListener("load", function onLoad(aEvent) {
-      cwindow.removeEventListener("load", onLoad);
-      let sbWidth = {};
-      try {
-        utils.getScrollbarSize(true, sbWidth, {});
-      } catch(e) {
-        Cu.reportError("Could not sample scrollbar size: " + e + " -- " +
-                       e.stack);
-        sbWidth.value = 0;
-      }
-      // Minimum width of 10 so that we have enough padding:
-      sbWidth.value = Math.max(sbWidth.value, 10);
-      deferred.resolve(sbWidth.value);
-      iframe.remove();
+    return new Promise(resolve => {
+      cwindow.addEventListener("load", function onLoad(aEvent) {
+        cwindow.removeEventListener("load", onLoad);
+        let sbWidth = {};
+        try {
+          utils.getScrollbarSize(true, sbWidth, {});
+        } catch(e) {
+          Cu.reportError("Could not sample scrollbar size: " + e + " -- " +
+                         e.stack);
+          sbWidth.value = 0;
+        }
+        // Minimum width of 10 so that we have enough padding:
+        sbWidth.value = Math.max(sbWidth.value, 10);
+        resolve(sbWidth.value);
+        iframe.remove();
+      });
     });
-
-    return deferred.promise;
   }
 };
 Object.freeze(this.ScrollbarSampler);