diff --git a/browser/base/content/browser.js b/browser/base/content/browser.js
index 4661beba3e13..f7dfb987250e 100644
--- a/browser/base/content/browser.js
+++ b/browser/base/content/browser.js
@@ -115,8 +115,9 @@ var gEditUIVisible = true;
 
 __defineGetter__("gPrefService", function() {
   delete this.gPrefService;
-  return this.gPrefService = Cc["@mozilla.org/preferences-service;1"].
-                             getService(Ci.nsIPrefBranch2);
+  return this.gPrefService = Cc["@mozilla.org/preferences-service;1"]
+                               .getService(Ci.nsIPrefBranch2)
+                               .QueryInterface(Ci.nsIPrefService);
 });
 
 __defineGetter__("PluralForm", function() {
@@ -1526,7 +1527,7 @@ function initializeSanitizer()
     gPrefService.clearUserPref(kDidSanitizeDomain);
     // We need to persist this preference change, since we want to
     // check it at next app start even if the browser exits abruptly
-    gPrefService.QueryInterface(Ci.nsIPrefService).savePrefFile(null);
+    gPrefService.savePrefFile(null);
   }
 
   /**