I just installed the new Firefox 4.0 beta 1 and looks nice.
They improved the CSS3 support with also CSS transitions.
Support also for the HTML5 HD video WebM.
New Tab Location
The tabs are on top but you can put them back down if you want. Persoanlly I like the new position 😉
Switch to Tab
A nice feature wich allows you to jump to an opened tab.
I'm using Windows XP so I can not see the button. It only shows on Windows 7 and Vista.
New Add-Ons Manager
Super-cool Add-ons, Themes and Extension manager.
You can send feedback to Firefox using the button on the left side.
Are you going to try Firefox 4.0 beta?