I'm sorry but this is near unusable since the death of XUL and introduction of WebExtensions. I was a truly avid user of VimFX which did the same thing. Vimium these days does not work when a page hasn't finished loading and it doesn't work on blank pages (about:blank) or any other "system page" (like the preferences or addons). The "o" key can no longer highlight the address bar but brings up a non-native address bar that does not find my bookmarks as it should.

Imagine you couldn't click on another tab with your mouse pointer while the current active one is loading. Yes, it's as terribly frustrating as it sounds.

In the XUL days I could even use vim shortcuts to access every button in the Firefox UI!

Luckily there is a solution for now. VimFx[1] is still being updated and works with the LegacyFox shim!