You might try Pentadactyl, a fork of Vimperator by the original authors.
Might be good to note that Pentadactyl doesn't currently work with FF15.
The Pentadactyl nightly works fine in Firefox 15
Those who are recommending Pentadactyl, can you explain why it's worth going through this installation hoop? I like to use FF beta (16 currently) but there is no Pentadactyl support. I use Vimperator.
There is a version of vimium for firefox too. I prefer using it so i don't have to think about the minor differences between vimperator and vimium when I switch browsers.
I found it very limited (just movement keys) even compared to Chrome vimium. You can remap vimperator (or pentadactyl) keys anyway. The only difference I remember was d (close tab) instead of x.
pentadactyl. try the nightly builds for ff15 compatibility