A Greasemonkey script template that injects the main script into the global scope, thus escaping the sandbox.

greasemonkey.template.js

// ==UserScript==
// @name        %name%
// @namespace   %namespace%
// @description %description%
// @include     %include%
// @exclude     %exclude%
// @version     1
// @noframes
// @grant       none
// @run-at      document-end
// ==/UserScript==

(main => {
    // http://stackoverflow.com/a/5006952/27862
    var script = document.createElement('script');
    script.textContent = `try {(${main})();} catch (e) {console.log(e);}`;
    document.body.appendChild(script).parentNode.removeChild(script);
})(() => {
    
    // script content here
    
});