Bug 479461 - Private browsing, close window, can't restore session; r=gavin a1.9.1=blocking-firefox3.1+

browser/components/privatebrowsing/src/nsPrivateBrowsingService.js

@@ -171,13 +171,19 @@ PrivateBrowsingService.prototype = {
 
         let browser = Cc["@mozilla.org/appshell/window-mediator;1"].
                       getService(Ci.nsIWindowMediator).
-                      getMostRecentWindow("navigator:browser").gBrowser;
+                      getMostRecentWindow("navigator:browser");
+
+        // if all browser windows are closed (i.e. on Mac) we won't have a window here
+        // this is only needed on Mac, but it's just good hygiene
+        if (browser) {
           // this ensures a clean slate from which to transition into or out of
           // private browsing
+          browser = browser.gBrowser;
           browser.addTab();
           browser.removeTab(browser.tabContainer.firstChild);
         }
       }
+    }
     else
       this._saveSession = false;
   },