This userscript demonstrates how a user script author can add a DOMContentLoaded listener to trigger their user script.

gmDOMContentLoadedExample.user.js

// ==UserScript==
// @name         DOMContentLoaded Issue (A Workaround Example)
// @namespace    gmTest
// @include      http://erikvold.com/tests/greasemonkey/DOMContentLoaded/index.cfm
// @include      http://erikvold.lan/tests/greasemonkey/DOMContentLoaded/index.cfm
// @version      0.1
// @author       Erik Vergobbi Vold
// @license      GPL version 3 or any later version; http://www.gnu.org/copyleft/gpl.html
// @description  This userscript demonstrates how a user script author can add a DOMContentLoaded listener to trigger their user script.
// ==/UserScript==

alert('user script level - load!');

document.addEventListener("DOMContentLoaded", function() {
  alert('user script level - DOMContentLoaded (use capture)');
}, true);
document.addEventListener("DOMContentLoaded", function() {
  alert('user script level - DOMContentLoaded (!use capture)');
}, false);
window.addEventListener("load", function() {
  alert('user script level - load (use capture)');
}, true);