I've been using Firefox as my main browser for a couple of years now. It's really getting better in every build. And Vimperator is not available for Chrome. Nowadays my biggest reason to stay with Firefox is that I trust Mozilla more than I trust Google.
a) The fact that they didn't appear in Snowden's documents like the other companies,
b) the fact that companies like Google (and Facebook, Yahoo, Microsoft, etc.) have it as their mission to build as precise user profiles as possible ("data hoover"), for example because the rely on advertising revenue. (Google is a 1-trick-pony, advertising being its trick.)
For the most part, Mozilla concentrates on releasing software rather than storing copious amounts of data about users. (Perhaps their "startup" Persona service is an exception to this, but I doubt it.) NSA is more likely to target all that juicy user data that Google has. They might also seek to undermine the security of software, but Mozilla aren't going to make that easy for them.
I switched from FF when it was sluggish.
Funny, the reason I don't switch back is the opposite of yours: I don't want to miss Vimium... how does Vimperator compare to Vimium?
I was in the same boat. I tried to switch back to Firefox a while back. Some people prefer Vimperator and Pentadactyl, but after using Vimium for a while, I found them to be a little too complicated and, well, different from what I was used to. It's possible I could have adjusted, but I didn't bother at the time and quickly returned to Chromium.
Later I discovered a Firefox addon called VimFX that basically aims to be "Chromium for Firefox": A very simple set of Vim-like shortcuts, configured in almost exactly the way Chromium is. Once I started using it, I felt right at home in Firefox, and I haven't looked back since. It's got a few warts (delay when using gg, Ctrl-F rebound to PageDown by default), but all things considered I'm pretty happy with it.
