It's most likely the plugins that were causing all your problems.
I keep one firefox window open 24/7 with about 20 tabs on it to monitor my sites, and other windows for surfing and shit. Never have any lock up or slow downs. Opteron 165 @ 2.7 might have something to do with that though
Memory usage can sometimes creep up close to 200megs but with 2gigs it doesn't matter.