I've been using Firefox 3b5 for a few weeks and have had zero problems. On my Mac, it's far improved from the 2.0.x builds, which would run out of memory after a day or two. On the other hand, I'm not using as many extensions (only AdBlock Plus, and Remember The Milk's Gmail plugin).

Also, for what it's worth, I use AdBlock Plus with a 100% hand-build list. I found that the official lists tended to cause too much collateral damage, particularly at the New York Times. I've even used AdBlock Plus's ability to nuke DOM elements in a few cases, although I suppose other plugins can do that better.