Yes, Chrome is faster, but speed isn't everything...for example, I can't use Chrome at work since I usually have 15 tabs or so open all the time, and if I was using Chrome I couldn't read what was on the tabs so it would be useless to me. Firefox has a scrolling tab bar, so you can always identify tabs. Links are underlined in Chrome, no way to turn that off if you care (I do *smile*). And there are various other more serious usability issues with Chrome that Firefox has dealt with nicely. Ad blocking is another one, where ads aren't really blocked on Chrome, they're removed after the fact...Firefox actually allows blocking them, so you don't see empty spaces where they would have been generally.
I think that you could summarize this situation by saying that Firefox creators care a good bit more about usability than Google ever has. Google focuses on getting something out, doesn't care a fig about usability issues.