--- a/toolkit/components/extensions/ExtensionChild.jsm
+++ b/toolkit/components/extensions/ExtensionChild.jsm
@@ -39,16 +39,17 @@ Cu.import("resource://gre/modules/Extens
 
 const {
   DefaultMap,
   EventEmitter,
   LimitedSet,
   defineLazyGetter,
   getMessageManager,
   getUniqueId,
+  getWinUtils,
   withHandlingUserInput,
 } = ExtensionUtils;
 
 const {
   EventManager,
   LocalAPIImplementation,
   LocaleData,
   NoCloneSpreadArgs,
@@ -679,18 +680,17 @@ class ProxyAPIImplementation extends Sch
 
   callFunctionNoReturn(args) {
     this.childApiManager.callParentFunctionNoReturn(this.path, args);
   }
 
   callAsyncFunction(args, callback, requireUserInput) {
     if (requireUserInput) {
       let context = this.childApiManager.context;
-      let winUtils = context.contentWindow.getInterface(Ci.nsIDOMWindowUtils);
-      if (!winUtils.isHandlingUserInput) {
+      if (!getWinUtils(context.contentWindow).isHandlingUserInput) {
         let err = new context.cloneScope.Error(`${this.path} may only be called from a user input handler`);
         return context.wrapPromise(Promise.reject(err), callback);
       }
     }
     return this.childApiManager.callParentAsyncFunction(this.path, args, callback);
   }
 
   addListener(listener, args) {

--- a/toolkit/components/extensions/ExtensionCommon.jsm
+++ b/toolkit/components/extensions/ExtensionCommon.jsm
@@ -41,16 +41,17 @@ var {
   DefaultWeakMap,
   EventEmitter,
   ExtensionError,
   defineLazyGetter,
   filterStack,
   getConsole,
   getInnerWindowID,
   getUniqueId,
+  getWinUtils,
 } = ExtensionUtils;
 
 XPCOMUtils.defineLazyGetter(this, "console", getConsole);
 
 var ExtensionCommon;
 
 /**
  * A sentinel class to indicate that an array of values should be
@@ -663,19 +664,17 @@ class LocalAPIImplementation extends Sch
   callFunctionNoReturn(args) {
     this.pathObj[this.name](...args);
   }
 
   callAsyncFunction(args, callback, requireUserInput) {
     let promise;
     try {
       if (requireUserInput) {
-        let winUtils = this.context.contentWindow
-                           .getInterface(Ci.nsIDOMWindowUtils);
-        if (!winUtils.isHandlingUserInput) {
+        if (!getWinUtils(this.context.contentWindow).isHandlingUserInput) {
           throw new ExtensionError(`${this.name} may only be called from a user input handler`);
         }
       }
       promise = this.pathObj[this.name](...args) || Promise.resolve();
     } catch (e) {
       promise = Promise.reject(e);
     }
     return this.context.wrapPromise(promise, callback);

--- a/toolkit/components/extensions/ext-tabs-base.js
+++ b/toolkit/components/extensions/ext-tabs-base.js
@@ -1201,18 +1201,16 @@ class WindowTrackerBase extends EventEmi
 
     this._listeners = new DefaultMap(() => new Set());
 
     this._statusListeners = new DefaultWeakMap(listener => {
       return new StatusListener(listener);
     });
 
     this._windowIds = new DefaultWeakMap(window => {
-      window.QueryInterface(Ci.nsIInterfaceRequestor);
-
       return getWinUtils(window).outerWindowID;
     });
   }
 
   isBrowserWindow(window) {
     let {documentElement} = window.document;
 
     return documentElement.getAttribute("windowtype") === "navigator:browser";

--- a/toolkit/components/extensions/ext-theme.js
+++ b/toolkit/components/extensions/ext-theme.js
@@ -6,16 +6,20 @@ Cu.import("resource://gre/modules/Servic
 
 XPCOMUtils.defineLazyModuleGetter(this, "LightweightThemeManager",
                                   "resource://gre/modules/LightweightThemeManager.jsm");
 
 XPCOMUtils.defineLazyGetter(this, "gThemesEnabled", () => {
   return Services.prefs.getBoolPref("extensions.webextensions.themes.enabled");
 });
 
+var {
+  getWinUtils,
+} = ExtensionUtils;
+
 const ICONS = Services.prefs.getStringPref("extensions.webextensions.themes.icons.buttons", "").split(",");
 
 /** Class representing a theme. */
 class Theme {
   /**
    * Creates a theme instance.
    *
    * @param {string} baseURI The base URI of the extension, used to
@@ -38,19 +42,17 @@ class Theme {
    *
    * @param {Object} details Theme part of the manifest. Supported
    *   properties can be found in the schema under ThemeType.
    * @param {Object} targetWindow The window to apply the theme to. Omitting
    *   this parameter will apply the theme globally.
    */
   load(details, targetWindow) {
     if (targetWindow) {
-      this.lwtStyles.window = targetWindow
-        .QueryInterface(Ci.nsIInterfaceRequestor)
-        .getInterface(Ci.nsIDOMWindowUtils).outerWindowID;
+      this.lwtStyles.window = getWinUtils(targetWindow).outerWindowID;
     }
 
     if (details.colors) {
       this.loadColors(details.colors);
     }
 
     if (details.images) {
       this.loadImages(details.images);