I downloaded the recently released Firefox 3 Beta 5 to have a look last week. It's been rumoured that the performance when downloading pages has been improved. Based on my unscientific experiments, this is indeed the case. It also appears to run better than v2 on low-end PC's.
One of the things I like though, is the thought that has gone into the UI when running on the various different Operating Systems. Take the following screenshots as an example:
(Running on Windows XP with the Zune theme)
(Running on Vista with Aero Basic)
As you can see, the icons etc have been optimised to suit the platform. This is a very nice touch that aids integration with the OS and so will help user acceptance.