The website seems to be very slow which causes the images to load not at all or really slowly.
I cannot even get the site to load at all anymore + greasemonkey is useless on latest Firefox.
Tampermonkey works fine on Firefox. Upgraded just a bit before the new version came out and had no issues since then. Haven't tested this script in particular on TM though.
I have had zero luck with it on Firefox Quantum. Looks like tampermonkey will require an entire rewrite
https://www.ghacks.net/2017/09/21/greasemonkey-4-firefox-57-compatibility/Not a big fan of most "upgrades" but this one has been impressive so far. Hate to lose ability to use bitcointalk++ though, had a nice old-school vibe to it. For now Cyrus' dark CSS at least tones down the brightness.
Correct me if I'm wrong but I think you're mixing up Greasemonkey (the AFAIK original userscript manager plugin) with Tampermonkey (a relatively new and IMO better plugin, written based on the WebExtension standard which sidesteps any incompatibilities a massive browser upgrade might bring).
AFAIK it uses the same GM_* API (with a few extra functions) so any Gresemonkey specific functions that BT++ uses (looking at the source, it seems to only use GM_xmlhttpRequest) should work on TM.