Not sure if it will help, but have you tried running Firefox's hidden profile manager and created a new profile? Firefox was horrible to run on OS X for me, it would drag the whole system down, but since changing to a new profile it has run like a dream and not reverted back.
Creating a new FF profile made a significant difference for me, too, when I did it recently (for the first time in a couple years). I suspect that will be true on any OS, if you do a fair amount of installing and removing extensions, user scripts, etc.
