>>778
↓これでどうでしょ？

// Undo List In Tab Context Menu
(function(){
var tm = document.getAnonymousElementByAttribute(gBrowser,'anonid','tabContextMenu');
var menu = document.getElementById('historyUndoMenu');
var max = tm.childNodes.length;
var after;
for(var i = 0;i < max;++i){
after = tm.childNodes[i];
if(after.id == 'tabContextUndoCloseTab'){ after = tm.childNodes[++i]; break; }
}
after.parentNode.insertBefore(menu,after);
document.getElementById('endUndoSeparator').style.display = 'none';
menu.parentNode.addEventListener('popupshowing',function(){ menu.setAttribute('disabled',false); },false);
})();