It's been a while since we last talked about Firefox, and having just updated to the FF 4.0 Beta 12 I thought I'd revive this thread long enough to share my opinions.
The latest beta is running a hell of a lot smoother than the old FF 3.xx I was using, and using less memory at just under 120MB (3.xx was ~130). Loading is faster and generally the functions all seem sped up, not that I had any real problem with the old version.
I'm not sure I like having the tabs above the address and toolbars but I'll keep with it for now and see if it grows on me. A few minor differences in how bookmarks and history are displayed but nothing groundbreaking there.
The personas are next to useless now with only a sliver of space at the top for anything to show. I haven't played with the appearance settings much but I can't see where (or why, come to think of it) you can add space for more pretty pictures around the workspace.
All the option tabs are now combined in to one Firefox tab at the top and I definitely like this. It feels natural to point to one spot and have access to all the options.
Anyhow, I can wholly recommend updating to the beta for most people. I believe there's enough of an improvement to make it worthwhile. Just check the Firefox website for known issues to make sure they don't affect you. You can also degrade back to the FF 3.6 if it's not your thing.