gistfile1.js

// create XUL element
function createToolbarButton() {
    const XUL_NS = "http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul";
    var anitem = document.createElementNS(XUL_NS, "toolbarbutton");
    anitem.setAttribute("id", "Testing-Doit-Button2");
    anitem.setAttribute("class", "toolbarbutton-1 chromeclass-toolbar-additional");
    anitem.setAttribute("label", "label");
    anitem.setAttribute("tooltiptext", "Do it!");
    anitem.setAttribute("oncommand", "testing_doit();");
    return anitem;
}
function placeToolbarButton() {
    var bar = document.getElementById("nav-bar");
    var newitem = createToolbarButton();
    bar.appendChild(newitem);
}
placeToolbarButton();