This isn't going to help some of the people in this thread, but I just went through a massive browser foul-up with my long-in-the-tooth Thinkpad, XP, and the very latest release of Firefox.
Crimresearch. Yours sounds the most like what happened to me. Whenever the browser had been running for a while after boot-up nearly everything would stall it. And it wasn't a very long while. I'm used to needing to reboot for Firefox. It's a resource hog on my ancient machine, but that was every couple of days or so. So I'd do a re-boot and run CCleaner to zero out temp files. This was showing up in a few hours, and was
much worse.
I had also gotten a recent MS auto-update for XP, and switched from Open Office to Libre, so there were a lot of potential culprits.
I cleaned up everything I could find in start-up that looked expendable (not much, I don't let much stay in there), let CCleaner check out the registry (no issues), and disabled all the Firefox plug-ins and extensions that I thought might be screwing things up (no joy).
After snooping around on the web for a while I learned that FF v.13.0.1 had started setting "Smooth Scrolling" turned on by default. Quite a few people were reporting similar problems.
I turned it off. (Tools --> Options --> Advanced --> uncheck "Use Smooth Scrolling".)
Everything got better, or as much better as I can expect from a seven year old R51 Thinkpad on XP.
If you have a cranky,
old 
p) machine, and things went all to hell after your last Firefox update, this might be worth a look.
