toggleMenubarHotkey.uc.js

// ==UserScript==
// @name           Toggle Menubar Hotkey
// @version        1.1.1
// @author         aminomancer
// @homepage       https://github.com/aminomancer
// @description    Press alt+M to toggle the menubar.
// @license        This Source Code Form is subject to the terms of the Creative Commons Attribution-NonCommercial-ShareAlike International License, v. 4.0. If a copy of the CC BY-NC-SA 4.0 was not distributed with this file, You can obtain one at http://creativecommons.org/licenses/by-nc-sa/4.0/ or send a letter to Creative Commons, PO Box 1866, Mountain View, CA 94042, USA.
// ==/UserScript==

(function () {
    const hotkeyId = "key_toggleMenubar";
    let hotkeyRegistered = _ucUtils.registerHotkey(
        {
            id: hotkeyId,
            modifiers: "alt",
            key: "M",
        },
        (win, key) => {
            if (win === window)
                Services.obs.notifyObservers(
                    null,
                    "browser-set-toolbar-visibility",
                    JSON.stringify([
                        CustomizableUI.AREA_MENUBAR,
                        AutoHideMenubar._node.getAttribute("inactive"),
                    ])
                );
        }
    );

    function init() {
        if (!hotkeyRegistered) return;
        document.getElementById("toolbar-menubar").setAttribute("key", hotkeyId);
        let src = onViewToolbarsPopupShowing.toSource();
        if (src.startsWith("function"))
            eval(
                `window.onViewToolbarsPopupShowing = function uc_onViewToolbarsPopupShowing ` +
                    src
                        .replace(/^function onViewToolbarsPopupShowing/, "")
                        .replace(
                            /if \(popup\.id != \"toolbar-context-menu\"\)/,
                            `if (toolbar.hasAttribute("key"))`
                        )
            );
    }

    if (gBrowserInit.delayedStartupFinished) {
        init();
    } else {
        let delayedListener = (subject, topic) => {
            if (topic == "browser-delayed-startup-finished" && subject == window) {
                Services.obs.removeObserver(delayedListener, topic);
                init();
            }
        };
        Services.obs.addObserver(delayedListener, "browser-delayed-startup-finished");
    }
})();