[Jach]

It's less of a problem on Firefox because you aren't forced into auto-updating them. But yeah, Stylish is the biggest example that comes to mind.

I prefer using Greasemonkey / Tampermonkey but the ecosystem is full of sketchy scripts too and some people foolishly have auto-updates enabled. Also it's bizarrely really hard to get someone to use such a user script if they don't already have the parent extension installed, but if you package it as an extension on its own they'll try it much more easily.