>>332
CustomizableUI.createWidget使わないと無理

(function () {
try {
const iconSvg = `<svg xmlns="http://www.w3.org/2000/svg"; fill="deepskyblue" viewBox="0 0 24 24">
<path d="M12 6V2L8 6l4 4V7c3.31 0 6 2.69 6 6a6 6 0 0 1-6 6c-2.87 0-5.26-2.01-5.83-4.65H5.06c.6 4 4.04 7.1 8.07 7.1 4.42 0 8-3.58 8-8s-3.58-8-8-8z"/>
</svg>`;
CustomizableUI.createWidget({
id: 'restart-button',
type: 'custom',
defaultArea: CustomizableUI.AREA_NAVBAR,
onBuild: function(aDocument) {
const btn = aDocument.createXULElement('toolbarbutton');
btn.id = "restart-button";
btn.setAttribute("class", "toolbarbutton-1 chromeclass-toolbar-additional");
btn.setAttribute("label", "再起動");
btn.setAttribute("tooltiptext", "Firefoxを再起動します");
btn.style.listStyleImage = `url('data:image/svg+xml;charset=utf-8,${encodeURIComponent(iconSvg)}')`;
btn.addEventListener('command', () => Services.startup.quit(Ci.nsIAppStartup.eRestart | Ci.nsIAppStartup.eAttemptQuit));
return btn;
}
});
} catch(e) { };
})();