--- a/browser/base/content/browser-fullZoom.js
+++ b/browser/base/content/browser-fullZoom.js
@@ -256,31 +256,31 @@ var FullZoom = {
     menuItem.setAttribute("checked", !ZoomManager.useFullZoom);
   },
 
   // Setting & Pref Manipulation
 
   /**
    * Reduces the zoom level of the page in the current browser.
    */
-  reduce: function FullZoom_reduce() {
+  async reduce() {
     ZoomManager.reduce();
     let browser = gBrowser.selectedBrowser;
     this._ignorePendingZoomAccesses(browser);
-    this._applyZoomToPref(browser);
+    await this._applyZoomToPref(browser);
   },
 
   /**
    * Enlarges the zoom level of the page in the current browser.
    */
-  enlarge: function FullZoom_enlarge() {
+  async enlarge() {
     ZoomManager.enlarge();
     let browser = gBrowser.selectedBrowser;
     this._ignorePendingZoomAccesses(browser);
-    this._applyZoomToPref(browser);
+    await this._applyZoomToPref(browser);
   },
 
   /**
    * Sets the zoom level for the given browser to the given floating
    * point value, where 1 is the default zoom level.
    */
   setZoom(value, browser = gBrowser.selectedBrowser) {
     ZoomManager.setZoomForBrowser(browser, value);
@@ -365,24 +365,27 @@ var FullZoom = {
    * prefs store.
    *
    * @param browser  The zoom of this browser will be saved.  Required.
    */
   _applyZoomToPref: function FullZoom__applyZoomToPref(browser) {
     if (!this.siteSpecific ||
         gInPrintPreviewMode ||
         browser.isSyntheticDocument)
-      return;
+      return null;
 
-    this._cps2.set(browser.currentURI.spec, this.name,
-                   ZoomManager.getZoomForBrowser(browser),
-                   this._loadContextFromBrowser(browser), {
-      handleCompletion: () => {
-        this._isNextContentPrefChangeInternal = true;
-      },
+    return new Promise(resolve => {
+      this._cps2.set(browser.currentURI.spec, this.name,
+                     ZoomManager.getZoomForBrowser(browser),
+                     this._loadContextFromBrowser(browser), {
+        handleCompletion: () => {
+          this._isNextContentPrefChangeInternal = true;
+          resolve();
+        },
+      });
     });
   },
 
   /**
    * Removes from the content prefs store the zoom level of the given browser.
    *
    * @param browser  The zoom of this browser will be removed.  Required.
    */

--- a/browser/base/content/test/general/browser_bug419612.js
+++ b/browser/base/content/test/general/browser_bug419612.js
@@ -5,17 +5,17 @@ function test() {
     let testPage = "http://example.org/browser/browser/base/content/test/general/dummy_page.html";
     let tab1 = BrowserTestUtils.addTab(gBrowser);
     await FullZoomHelper.selectTabAndWaitForLocationChange(tab1);
     await FullZoomHelper.load(tab1, testPage);
 
     let tab2 = BrowserTestUtils.addTab(gBrowser);
     await FullZoomHelper.load(tab2, testPage);
 
-    FullZoom.enlarge();
+    await FullZoom.enlarge();
     let tab1Zoom = ZoomManager.getZoomForBrowser(tab1.linkedBrowser);
 
     await FullZoomHelper.selectTabAndWaitForLocationChange(tab2);
     let tab2Zoom = ZoomManager.getZoomForBrowser(tab2.linkedBrowser);
     is(tab2Zoom, tab1Zoom, "Zoom should affect background tabs");
 
     Services.prefs.setBoolPref("browser.zoom.updateBackgroundTabs", false);
     await FullZoom.reset();