Per blake, because mac doesn't hae that object. r=ben sr=blake

xpfe/global/resources/content/globalOverlay.js

@@ -39,7 +39,7 @@ function goQuitApplication()
         return false;
     }
   };
-  if (!nativeAppSupport.isServerMode)
+  if (!nativeAppSupport || !nativeAppSupport.isServerMode)
     appShell.Quit();
   return true;
 }