>>750
setTimeout(function() {
gBrowser.mStrip.childNodes[1].appendChild(document.createElement("menuseparator"));
gBrowser.mStrip.childNodes[1].appendChild(menuitem1);
gBrowser.mStrip.childNodes[1].appendChild(menuitem2);
}, 0);
を
var tabContextMenu =
document.getAnonymousElementByAttribute(gBrowser, "anonid", "tabContextMenu")
|| document.getAnonymousElementByAttribute(document.getElementById("tabbrowser-tabs"), "anonid", "tabContextMenu");

tabContextMenu.appendChild(document.createElement("menuseparator"));
tabContextMenu.appendChild(menuitem1);
tabContextMenu.appendChild(menuitem2);
とするとか