Have you tried VimFx? It's one of the first addons I download (other than uBlock, HTTPS Everywhere, and RES). I like that it's lighter than Pentadactyl/Vimperator and it doesn't break Firefox's default behavior.

Chrome users can download Viumium[1] which does the exact same thing for Chrome.

0: https://addons.mozilla.org/en-us/firefox/addon/vimfx/ 1: https://vimium.github.io/