Chrome works just fine, and my Greasemonkey script, if copy/pasted into Firefox's Firebug, works just fine on these pages but I can't seem to get it automatically execute.
Download & install the script: http://sente.cc/js/testing_greasemonkey.user.js
and then go to https://i.stack.imgur.com/MK8ve.jpg (observe simple alert) and https://i.stack.imgur.com/q0tbI.jpg (observe no alert)
the script:
// ==UserScript==
// @name          Testing Greasemonkey
// @namespace     stuartpowers
// @description   Testing Greasemonkey
// @author        Stuart Powers
// @homepage      http://sente.cc/
// @include       http://i.imgur.com/*
// @include       http://imgur.com/*
// ==/UserScript==
// load https://i.stack.imgur.com/MK8ve.jpg (works)
// load https://i.stack.imgur.com/q0tbI.jpg (doesn't work)
(function () {
    alert("worked");
})()
 
    