I'm having the same problem and using all my cpu and memory, causing my computer to over heat and shutdown through out the day. I started having this problem about two releases ago and now the problem is even worse. Thinking of dumping FF this week.

go back to 3.6.12 or dump it

bold textAfter trying dozens of remedies and restoring my computer at least 10 times going back to january 15th, I finally found my answer: I had a desktop photo on my monitor which was slowing down the computer. I changed my Refresh rate in the Display Properties from 65 to 75 and it solved my problem.