Menu example for Custom Buttons https://addons.mozilla.org/addon/custom-buttons/ Simplified version of https://gist.github.com/Infocatcher/3137247

cbMenu.js

// https://gist.github.com/Infocatcher/5891622
var menu = [
	{ label: "Alert!", cb_id: "alert" },
	{ label: "Google", cb_id: "google", image: "https://www.google.com/favicon.ico" },
	"menuseparator",
	[
		"Menu label",
		{ label: "Sub item 1", cb_id: "sub" },
		{ label: "Sub item 2", cb_id: "sub2" }
	],
	[
		"Menu label 2",
		"https://www.google.com/favicon.ico",
		{ label: "Sub item 3", cb_id: "sub3" },
		{ label: "Sub item 4", cb_id: "sub4" }
	]
];

function parseItem(arr, parent) {
	if(!parent)
		parent = document.createElement("menupopup");
	arr.forEach(function(item) {
		var node;
		if(item == "menuseparator")
			node = document.createElement("menuseparator");
		else if(Array.isArray(item)) {
			node = document.createElement("menu");
			node.setAttribute("label", item[0]);
			var icon = item[1];
			if(typeof icon == "string") {
				node.className = "menu-iconic";
				node.setAttribute("image", icon);
			}
			var mp = document.createElement("menupopup");
			parseItem(item, mp);
			node.appendChild(mp);
		}
		else if(typeof item == "object" && item) {
			node = document.createElement("menuitem");
			if(item.hasOwnProperty("image") && !item.hasOwnProperty("class"))
				item.class = "menuitem-iconic";
			for(var attr in item) if(item.hasOwnProperty(attr))
				node.setAttribute(attr, item[attr]);
		}
		node && parent.appendChild(node);
	});
	return parent;
}

var mp = parseItem(menu);
mp.setAttribute("oncommand", "this.parentNode.handleCommand(event);");
mp.setAttribute("onclick", "if(event.button != 0) this.parentNode.handleCommand(event);");
mp.setAttribute("oncontextmenu", "return false;");

this.type = "menu";
this.orient = "horizontal";
this.appendChild(mp);

this.handleCommand = function(e) {
	var trg = e.target;
	var cmd = trg.getAttribute("cb_id");
	if(!cmd)
		return;
	var btn = e.type == "click" && e.button || 0;
	btn && closeMenus(trg); // See chrome://browser/content/utilityOverlay.js
	alert("Button: " + btn + "\nCommand: " + cmd);
};