Null out browser's focusedWindow field as well (to prevent leaks). b=283063 r=mconnor

browser/base/content/tabbrowser.xml

@@ -1139,7 +1139,16 @@
             // clean up the before/afterselected attributes before removing the tab
             oldTab.selected = false;
 
-            // XXX browser's destructor isn't always called, so we force a cleanup ourselves
+            // Because of the way XBL works (fields just set JS
+            // properties on the element) and the code we have in place
+            // to preserve the JS objects for any elements that have
+            // JS properties set on them, the browser element won't be
+            // destroyed until the document goes away.  So we force a
+            // cleanup ourselves.
+            // This has to hapen before we remove the child so that the
+            // XBL implementation of nsIObserver still works.  But
+            // clearing focusedWindow happens below because it gets
+            // reset by updateCurrentBrowser.
             oldBrowser.destroy();
             
             this.mTabContainer.removeChild(oldTab);
@@ -1149,6 +1158,9 @@
             this.mPanelContainer.selectedIndex = newIndex;
             
             this.updateCurrentBrowser();
+
+            // see comment above destroy above
+            oldBrowser.focusedWindow = null;
             }
           ]]>
         </body>