There are some new images, well mock ups, of the next generation of how Firefox (4.0) could look up on the FireFox developer's Blog.
The new shots show some interesting new features and some UI improvements. The first thing you notice is the more transparent window, this includes the tabs. Speaking of tabs they are showing both styles, on top and tradtional. Just to give you a feel for both of them.
Other UI improvements include a combination refresh, go, stop button.
There is not much else to go on and nothing that says this is how the new FireFox will really look, but hopefully the next version can work out some of the odd bugs that 3.5 has while maintaining security.
After all a cool looking, fast browser is not as important as one that works and is secure; well to me anyway