I know you might not be interested in anything chromium-based since this thread is about firefox, but qutebrowser[1] is a great solution for anybody looking for vimium/pentadactyl-like experience.
The only reason I switched away from Vivaldi was the sluggish UI (too many things on the main thread, I guess), but I hear there have been improvements in that area. I may give it another try.
[1] https://qutebrowser.org/