One of the speedbumps for Firefox right now is what happens when TraceMonkey can't trace (check out this post to find out more about what tracing is). Firefox falls back to its basic interpreter when JIT compilation isn't possible, and it's just not all that fast.
Mandelin's post is an excellent read if you're interested in digging deeper. The short version: Firefox will burn even hotter once JägerMonkey is on board. Keep an eye on the Firefox nightly builds -- it'll likely appear there first.