appMenuAboutConfigButton.uc.js

// ==UserScript==
// @name           App Menu about:config Button
// @version        1.2
// @author         aminomancer
// @homepage       https://github.com/aminomancer/uc.css.js
// @description    Adds an about:config shortcut button to the main app menu panel, under the built-in Settings button. It can open the built-in about:config page, or it can open the old-school about:config page with earthlng's [aboutconfig](https://github.com/earthlng/aboutconfig) module. To use that with fx-autoconfig, download ONLY the profile/chrome/utils/aboutconfig folder and place it inside your profile/chrome/resources folder. Then open config.xhtml and find & replace "userchromejs" with "userchrome" and save. Now "chrome://userchrome/content/aboutconfig/config.xhtml" should be the correct URL. By default the script will open to that link, so if you don't have that module installed the button will open to a blank page. If you can't get the module to work or if you just prefer Firefox's built-in page, you can change the constant on line 10 below to "about:config" and it'll open to the same page you'd get if you typed about:config in the address bar. (the URL must be in quotes) That said, typing about:config is already easy enough. The reason I made this script was to make a clean shortcut to reach the old-school page, and in a more central location than a bookmark. FYI I added an icon for this button (and for all the other main app menu buttons too) in uc-app-menu.css
// ==/UserScript==

(function () {
    // user configuration
    const config = {
        urlOverride: "",
        /* the script tries to automatically find earthlng's aboutconfig URL,
        and if it can't be found, uses the built-in about:config URL instead.
        if it's unable to find the URL for your particular setup, or if you just want to use the vanilla about:config page,
        replace this empty string with your preferred URL, in quotes.
        if you want to use my about:cfg script that registers earthlng's aboutconfig page to about:cfg,
        and you want the about:config button to take you to about:cfg, then leave this empty.
        it will automatically use about:cfg if the script exists.
        if about:cfg doesn't work for you then change the urlOverride in *that* script instead of this one. */
    };

    let { interfaces: Ci, manager: Cm } = Components;
    if (!FileUtils)
        ChromeUtils.defineModuleGetter(this, "FileUtils", "resource://gre/modules/FileUtils.jsm");

    function findAboutConfig() {
        if (config.urlOverride)
            // url overridden @line 12
            return config.urlOverride;
        if (
            Cm.QueryInterface(Ci.nsIComponentRegistrar).isContractIDRegistered(
                "@mozilla.org/network/protocol/about;1?what=cfg"
            )
        )
            // registerAboutConf.uc.js
            return "about:cfg";
        if (FileUtils.getDir("UChrm", ["resources", "aboutconfig", "config.xhtml"]).exists())
            // fx-autoconfig
            return "chrome://userchrome/content/aboutconfig/config.xhtml";
        if (FileUtils.getDir("UChrm", ["utils", "aboutconfig", "config.xhtml"]).exists())
            // earthlng's loader
            return "chrome://userchromejs/content/aboutconfig/config.xhtml";
        if (FileUtils.getDir("UChrm", ["utils", "aboutconfig", "aboutconfig.xhtml"]).exists())
            // xiaoxiaoflood's loader
            return "chrome://userchromejs/content/aboutconfig/aboutconfig.xhtml";
        else return "about:config"; // no about:config replacement found
    }

    async function createButton() {
        const configStrings = await new Localization(["toolkit/about/config.ftl"], true); // get fluent file for AboutConfig page
        const advancedPrefsLabel = await configStrings.formatValue(["about-config-page-title"]); // localize the "Advanced Preferences" string
        const { mainView } = PanelUI;
        const doc = mainView.ownerDocument;
        const settingsButton =
            doc.getElementById("appMenu-settings-button") ??
            doc.getElementById("appMenu-preferences-button");
        const prefsButton = doc.createXULElement("toolbarbutton");

        prefsButton.preferredURL = findAboutConfig();
        for (const [key, val] of Object.entries({
            id: "appMenu-advanced-settings-button",
            class: "subviewbutton",
            label: advancedPrefsLabel,
            oncommand: `openTrustedLinkIn(this.preferredURL, gBrowser.currentURI.spec === AboutNewTab.newTabURL || gBrowser.currentURI.spec === HomePage.get(window) ? "current" : "tab")`,
        }))
            prefsButton.setAttribute(key, val);

        settingsButton.after(prefsButton); // place after the built-in "Settings" button
    }

    function init() {
        PanelMultiView.getViewNode(document, "appMenu-multiView").addEventListener(
            "ViewShowing",
            createButton,
            { once: true }
        );
    }

    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");
    }
})();