Visually, Firefox has new tab position, and support Aero for Windows 7. A nice to see features. Also support a new platform of HTML5. You can watch an HD movie in this the first Firefox Beta. And just wait for new features for best performance, secure, and great themes for Linux and OSX.
Now, I am using this beta version. All my extensions that are very useful. They are disabled because of incompatible. So this beta is not recommended for daily usage when we use many add on in version 3.6. Use it just for try and give any feed back to Mozilla for better development. And for Add On developer, we are waiting for your stuff.
Try Firefox BETA.