thimble_tamper.js

// ==UserScript==
// @name         New Userscript
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  try to take over the world!
// @author       You
// @match        https://mozillathimblelivepreview.net/bramble/production/dist/index.html?locale=en-US
// @grant        none
// ==/UserScript==

(function() {
    'use strict';
//     window.addEventListener('load', function () {
        // mjk's idea
        let sp = null;
        const intv = window.setInterval(function () {
            [ sp ] = document.getElementsByTagName('iframe');
            if (sp !== undefined) {
                window.clearInterval(intv);
                sp.onload = function () {
                    sp.src = sp.contentWindow.document.body.innerHTML;
                };
            }
        }, 500);
//     });
})();