There's Vim-Vixen [1] and Tridactyl [2] for Firefox Quantum. Both suffer from "you can't use the extension on all pages"-itis, and Tridactyl even manages to break a handful of web pages, but for me it was good enough to stop using Firefox ESR and Vimperator.

Disclaimer: a lot of Tridactyl's jank is due to me.

[1] https://addons.mozilla.org/en-US/firefox/addon/vim-vixen/

[2] https://addons.mozilla.org/en-US/firefox/addon/tridactyl-vim...