Mozilla is still doing something with the keyboard shortcuts API to make that more capable. I don't know what exactly they're doing, it won't be as powerful as Vimperator, but it should be a good bit more powerful than Vimium.

This (obviously) won't be in Firefox 57, but should be ready relatively soonish after. So, this is something that they have already been actively working on for a few months. I'm guessing 59 or 60 from how I've heard it described.