[jboynyc]

While not a full replacement, QuantumVim is better than nothing:

https://addons.mozilla.org/en-US/firefox/addon/quantumvim/

[The-Compiler]

QuantumVim hasn't seen any activity in the past year: https://github.com/shinglyu/QuantumVim

But there's Saka Key, Vimium-FF and Tridactyl.