Vimperator works well and I loved it in FF3, but it seems they've decided to move away from certain Vim-like behaviors to either appeal more to non-Vim users (why?) or to just be more user-friendly (there's no such thing).

I've discovered a fork of Vimperator, Pentadactyl, allegedly developed by most of the primary developers of Vimperator. The fork was apparently political. It is supposed to be more focused on being Vim-like and allegedly has better support for FF4 too.

So far I do prefer the way it seems to look and feel, but it's hard to say exactly. It feels just slightly clunky so I'm not sure yet which one to stick with... It's annoying having to learn the differences in configuration and behavior, though they do appear to be few (perhaps fewer than the Vimperator upgrade)...