Issue #1158 - Reinstate tabcontainer variable

- This was erroneously removed in commit 1f5194b5f1deb0f36b36ed886d94ce5f8b62ca9d because it is still being used by surrounding code.

application/basilisk/base/content/tabbrowser.xml

@@ -6928,6 +6928,8 @@
         document.getElementById("alltabs_undoCloseTab").disabled =
           SessionStore.getClosedTabCount(window) == 0;
 
+        var tabcontainer = gBrowser.tabContainer;
+
         // Listen for changes in the tab bar.
         tabcontainer.addEventListener("TabAttrModified", this, false);
         tabcontainer.addEventListener("TabClose", this, false);