backout a4a1307fc4e1 due to failure in browser_bug580956.js et al

browser/base/content/browser.js

@@ -8449,8 +8449,8 @@ function restoreLastSession() {
 var TabContextMenu = {
   contextTab: null,
   updateContextMenu: function updateContextMenu(aPopupMenu) {
-    this.contextTab = aPopupMenu.triggerNode.localName == "tab" ?
-                      aPopupMenu.triggerNode : gBrowser.selectedTab;
+    this.contextTab = document.popupNode.localName == "tab" ?
+                      document.popupNode : gBrowser.selectedTab;
     let disabled = gBrowser.visibleTabs.length == 1;
 
     // Enable the "Close Tab" menuitem when the window doesn't close with the last tab.