document.getElementById("context-addonnotification") is null
[在此错误处中断] document.getElementById("context-add...tion").hidden = gContextMenu.onImage;

（来自firebug）
点击第二行显示一个窗口
以下是内容
addonnotification.onFirefoxLoad = function(event) {
  document.getElementById("contentAreaContextMenu")
          .addEventListener("popupshowing", function (e){ addonnotification.showFirefoxContextMenu(e); }, false);
};

addonnotification.showFirefoxContextMenu = function(event) {
  // show or hide the menuitem based on what the context menu is on
  document.getElementById("context-addonnotification").hidden = gContextMenu.onImage;
};

window.addEventListener("load", addonnotification.onFirefoxLoad, false);

document.getElementById("context-addonnotification") is null
找不到id为context-addonnotificatio的DOM元素
确定上下文中有"context-addonnotification"
若确实有，注意脚本加载位置，放在最下边执行或者用window.load等待所有DOM文档加载完毕之后再执行代码

虽然我不一定看得懂，不过谢谢了。你一直很照顾我。。。