>>805, >>808
(function(){
var btn=document.getElementById("new-tab-button");
btn.removeAttribute("command");
btn.removeAttribute("oncommand");
com=document.createElement("command");
com.setAttribute("id","fukusei");
com.setAttribute("oncommand", "gBrowser.selectedTab=gBrowser.duplicateTab(gBrowser.mCurrentTab)");
btn.setAttribute("command", "fukusei");
btn.appendChild(com);
})();