The "installed" panel is a trainwreck, the only thing that it displays correctly is the number of installed styles for the current page. Some of them are always missing from the list (often enabled ones) and some from unrelated pages are visible. If I close and re-open the popup the list changes so it can't possibly be a case of too URL filters.

It's buggy and hardly works

Malware, do not install if you want to keep your data secure!

it works but the website is incredibly slow please fix it it takes like 10 minutes to load the site and it always gives bad gateway errors

I didn't install this ! It was installed by some devious means I am not aware of. I'm reporting this to Mozilla.

STOP: PRIVACY WARNING!

Install STYLUS instead.

This extension, Stylish, was once the industry standard, but it is now malware. STYLUS is maintained by an active and friendly community of developers, and does not send your browsing information to privacy thieves.

Super slow and doesn't work properly.