WebExtensions are not allowed to run at addons.mozilla.org by design:

https://developer.mozilla.org/en-US/Add-ons/WebExtensions/Your_first_WebExtension#Testing

// ==UserScript==
// @name Github - Color issues title red.
// @namespace http://tampermonkey.net/
// @version 0.1
// @description try to take over the world!
// @author You
// @match https://github.com/*
// @grant none
// ==/UserScript==

(function() {
'use strict';

var run = function(url) {
if (url.match(/https:\/\/github\.com\/.*\/.*\/issues\/.*/)) {
var element = document.querySelector(".js-issue-title");
element.style.color = "red";
}
};

run(location.href);
var pS = window.history.pushState;
var rS = window.history.replaceState;

window.history.replaceState = function(a, b, url) {
run(url);
rS.apply(this, arguments);
};
window.history.pushState = function(a, b, url) {
run(url);
pS.apply(this, arguments);
};
})();