Comment by The-Compiler

There's Vimium-FF and Saka Key at the moment:

https://addons.mozilla.org/en-GB/firefox/addon/vimium-ff/ https://key.saka.io/

Also, Tridactyl is at the planning stage and working with Mozilla on a better keyboard WebExtension API:

https://github.com/cmcaine/tridactyl https://bugzilla.mozilla.org/show_bug.cgi?id=1215061

edit: Oh, looks like Tridactyl has quite some code already. I'm not sure if it's runnable yet, though.