Yep, I’m using Firefox 4 (the Beta version) right now.
The new Firefox 4 is a new look. Google Chrome as well as Opera may have inspired the new Firefox look. The tabs are now located at the tab like Google Chrome. It also has the Windows Aero theme which looks great.
The new Firefox 4 also uses the Windows 7 taskbar options which include opening a new tab and Private Browsing.
This is very early beta so there are a few bugs here and there and I expect frequent crashes.
If you want to join the beta party, you can go to to this Mozilla link and check it out yourself.
Disclaimer: Use at your own risk. I wouldn’t recommend you use it as a primary browser unless you really want to test it out and send feedback to Mozilla.
Looks like Mozilla has posted the official Firefox 4 Beta. Check it out.