One of my main reasons for using firefox is Tridactyl [0].

It's the most comprehensive vi-mode addon for a mainstream browser. Support bookmarks, search engines, pagination, external text editing and colorschemes on top of the usual features something like vimium has. It is under active development [1] and written in TypeScript (which is also a plus for hackability in my book).

[0] https://addons.mozilla.org/en-US/firefox/addon/tridactyl-vim... [1] https://github.com/tridactyl/tridactyl