--- a/toolkit/content/globalOverlay.js
+++ b/toolkit/content/globalOverlay.js
@@ -11,28 +11,29 @@ function closeWindow(aClose, aPromptFunc
                      .getService(Components.interfaces.nsIWindowMediator);
   var e = wm.getEnumerator(null);
   
   while (e.hasMoreElements()) {
     var w = e.getNext();
     if (++windowCount == 2) 
       break;
   }
-
+  
   var inPrivateBrowsing = false;
-  try {
-    if (["@mozilla.org/privatebrowsing;1"] in Components.classes) {
-      var pbSvc = Components.classes["@mozilla.org/privatebrowsing;1"]
-                            .getService(Components.interfaces.nsIPrivateBrowsingService);
-      inPrivateBrowsing = pbSvc.privateBrowsingEnabled;
-    }
-  } catch(e) {
-    // safe to ignore
-  }
-
+  let chromeWin  = window.QueryInterface(Ci.nsIInterfaceRequestor)
+                         .getInterface(Ci.nsIWebNavigation)
+                         .QueryInterface(Ci.nsIDocShellTreeItem)
+                         .rootTreeItem
+                         .QueryInterface(Ci.nsIInterfaceRequestor)
+                         .getInterface(Ci.nsIDOMWindow)
+                         .wrappedJSObject;
+  
+  if (chromeWin != null) 
+    inPrivateBrowsing = chromeWin.gPrivateBrowsingUI.privateWindow;
+  
   // If we're down to the last window and someone tries to shut down, check to make sure we can!
   if (windowCount == 1 && !canQuitApplication("lastwindow"))
     return false;
   else if (windowCount != 1 || inPrivateBrowsing)
 #endif
     if (typeof(aPromptFunction) == "function" && !aPromptFunction())
       return false;