I've been trying out the new Firefox 1.5 beta 1 on my Mac for the last couple of days. I have to say, I like it. It works well. They even seem to have fixed the shutdown problem I was having on the Mac! (Basically, the old version of Firefox would NEVER shut down. It would stay stuck in memory forever).
Kudos to Mozilla for coming out with another great product! You gotta love open source software!