Just installed Firefox 4. It's UI seems significantly less antiquated as Firefox hadn't really changed its UI much from the when I started using it at Firebird 0.7 to Firefox 3.6.16, but it will take some getting used to. The tabs are at the top, Chrome-style. They also got rid of the venerable stop, go, reload buttons and replaced them with a single context sensitive button that changes depending on the page state. I like this, but hate how they put it on the right of the address bar as it is in IE. You can move it over to the left, but it seems to have special integration when it is on the right of the address bar that doesn't happen when it is on the left.
It also leverages Aero now on Vista and 7, but the window hitches when moving it around on my system. This does not occur for other applications on my system. That said, I do have a tab open with Netflix Instant, so that could be the source of that issue. Supposedly this version has improved performance, especially in Javascript (again). It also improves standards compliance, getting a 97/100 on the Acid 3 test (pretty impressive IMO).
I have read that a lot of people have returned to 3.6.x after using 4 for a short time because of bugs, freezing, and slowness but I haven't seen this so far myself. Besides, I figure 4.0.1 will be out in a week or two to correct the most widely reported or serious issues. Most of my add-ons from 3.6.16 work with 4, but admittedly I don't use very many. A couple of them had to be updated to new versions to work with 4.