The main issue is that Firefox has tons of memory leaks. Sadly, it's actually WORSE on Aurora, where you'd think they're working on the issue, than with the regular version. I frequently look up the memory usage, and after a couple hours' use: 1.5 gig in RAM, 1.something in virtual memory. Totally unacceptable, it just slows down to a crawl. Close down the leaks and this baby will run just fine. It's been two years (since v.3 came out) and they're making the problem worse, not better. Firefox 4 or 5 was briefly working fine but then by the next release they broke it again.

Regarding the add-ons, they're now running tests on them and pointing fingers to the worst offenders in terms of slowing down the browser. That's forced some people to patch up... If only they did the same thing on internal components of the browser.