GM_registerMenuCommand @run-at document-start

test.user.js

// ==UserScript==
// @name test
// @namespace test
// @include *
// @version 1
// @grant GM_registerMenuCommand
// @run-at document-start
// ==/UserScript==


(function () {
  GM_registerMenuCommand('Test1', function () {
    location.href = "https://getfirebug.com/";
  });
}());

window.addEventListener('load', function () {
  GM_registerMenuCommand('Test2', function () {
    location.href = "https://getfirebug.com/";
  });
});

setTimeout(function () {
  GM_registerMenuCommand('Test3', function () {
    location.href = "https://getfirebug.com/";
  });
}, 0)

GM_registerMenuCommand('Test4', function () {
  location.href = "https://getfirebug.com/";
});