Original Question (only about input boxes):
I use Arc-Dark theme on Ubuntu Gnome, and I know this issue existed on older Firefox versions too but it could be fixed with this Stylish script (which wasn't a great fix in itself as it screwed up the Downloads button).
Now the script doesn't work anymore, and I was thinking about reporting it to Mozilla, but they don't really have a GitHub repo for Firefox itself, only for separate components, so I don't really know where I should open the issue.
Is there a temporary fix for this? And could someone who knows how to properly do it report the issue to Mozilla or to the Gnome project (dunno what would be more appropriate)?
While in the answers you can find a temporary workaround for the input boxes, the same dark text on dark background issue seems to happen in dropdown menus (not all of them though, which is weird, may be related to different HTML/CSS techniques to create them), and since it's not much discussed but the two issues are most likely linked, I'm adding it to this question. The way to fix it is probably similar to the ones provided below, only with a different entry in
userContent.css, but it'd be nice if someone provided it here. I'm gonna edit my own answer if I have time to find it out myself.
It looks like even buttons are black-on-black on some websites. Here's an example, to see the buttons select "new program" on the left, paste this example program there, and this example query in the bottom right panel, then hit "Run!".