This addon is an excellent replacement to the now privacy-hostile Stylish addon. It has everything the original addon had and more, all without collecting your data on the sly.

But what really prompted me to write this review is the recent removal of the character limit on copying from the editor. This had bugged me so much in all the years I had used Stylish (and the short while I've used Stylus). I'm not sure when or why it was implemented, but it always felt like some weird DRM right there in the editor. Kudos to the Stylus team for taking that out. :D

EDIT: For people who want to style the Firefox UI and things like "about:" pages, you can use userChrome.css and userContent.css for those.