Firefox still supports mobile plugins but, a while ago (IIRC last year), the plugin API changed, as a result currently the selection of available plugins is much smaller than it used to be. I should add that I haven't checked in a few months, maybe it's improved now.
They didn't change the plugin API, they did a massive rewrite of the mobile browser engine infrastructure that broke most of the plugin APIs, and probably the test suites as well. So they are disabled until they can be fixed and proven to work.
I use dark reader on chrome for computers. They also have a safari extension that surprisingly works just as well.
https://github.com/darkreader/darkreader
https://darkreader.org/safari/
I'm concerned, though, whether these extensions work the same as chrome extensions.
Can they view and potentially upload your browser passwords/bank info/etc?
Is there some limitation that prevents this in the implementation?