[Bananaman28]

I can recommend Wasavi, which is a browser extension that allows you to use Vim commands in most text windows you can edit. https://github.com/akahuku/wasavi I use it on a daily basis and although it's not Vim it's still pretty good.

[bovine3dom]

There's also https://github.com/glacambre/firenvim which, while it isn't real vim, it's neovim which is pretty close ;)

[Seirdy]

Tridactyl [0] lets you open text fields in your preferred $EDITOR with a keybind, open YouTube/Vimeo/etc links in MPV, etc. on top of adding Vim navigation to Firefox.

This comment was written in a floating Neovim window opened by Tridactyl.

[0]: https://github.com/tridactyl/tridactyl/