FF has become a resource hog. I’m sick of it.
That has been a persistent problem with FF since about version 0.7. The Mozilla people claim it is the add-ons that are at fault, so they won't fix the problem.
Mozilla has improved it some in later additions, but it still eats up memory and won't release it.
I have tried several 'memory release' add-ons and programs with minimal results.
I did recently install a memory cleaning program that runs in the background and does seem to help:
http://www.pcwintech.com/cleanmem.