--- a/js/xpconnect/loader/XPCOMUtils.jsm
+++ b/js/xpconnect/loader/XPCOMUtils.jsm
@@ -404,17 +404,17 @@ this.XPCOMUtils = {
                                    aTransform = val => val)
   {
     // Note: We need to keep a reference to this observer alive as long
     // as aObject is alive. This means that all of our getters need to
     // explicitly close over the variable that holds the object, and we
     // cannot define a value in place of a getter after we read the
     // preference.
     let observer = {
-      QueryInterface: this.generateQI([Ci.nsIObserver, Ci.nsISupportsWeakReference]),
+      QueryInterface: XPCU_lazyPreferenceObserverQI,
 
       value: undefined,
 
       observe(subject, topic, data) {
         if (data == aPreference) {
           if (aOnUpdate) {
             let previous = this.value;
 
@@ -577,16 +577,18 @@ this.XPCOMUtils = {
     Object.defineProperty(aObj, aName, {
       value: aValue,
       enumerable: true,
       writable: false
     });
   },
 };
 
+var XPCU_lazyPreferenceObserverQI = XPCOMUtils.generateQI([Ci.nsIObserver, Ci.nsISupportsWeakReference]);
+
 XPCOMUtils.defineLazyModuleGetter(this, "Services",
                                   "resource://gre/modules/Services.jsm");
 
 XPCOMUtils.defineLazyServiceGetter(XPCOMUtils, "categoryManager",
                                    "@mozilla.org/categorymanager;1",
                                    "nsICategoryManager");
 
 /**