Firefox 3.6 beta coming next week
For the most part, Firefox 3.6 will look a lot like version 3.5 of the browser. Mozilla is holding off on a major visual refresh until Firefox 4.0 is released sometime next year. But the update should bring a better session restore, improved page rendering, and overall faster performance and better stability.
As you can see from the screenshot of Firefox 3.6 Alpha above, Mozilla has also built a new tab-switching feature into the browser which allows you to see previews of each page. In order to enable this feature, you'll need to futz around with the about:config menu.