10行で動いたわ

(function(){
var btn=gBrowser.tabContainer.mTabstrip.getElementsByClassName('tabs-newtab-button').item(0);
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);
})();