If you are seeing CPU performance degrade with time it is (likely) a symptom of memory fragmentation. I have not seen any rendering benchmarks where gecko fairs significantly worse than the others.

Not sure what the issue is but FF is using about 50% of my CPU.. steady. Other browsers I tried use much, much less (usually less than 5%).

Are you running a dual-core CPU? ;)

It is likely that the pages you view contain aggressive Javascript and Firefox doesn't do enough to tame it.

Yes, c2d. I care only because I hate fans coming on. I like my MBP silent.