There are a number of changes to the final version, mostly to how it looks. These include changes to Firefox 2's theme and user interface, toolbar buttons that glow when you hover over them.
There is also a better Windows installer which resolves many long-standing issues in the browser.
A big cheese in Mozzarella said that this version is more for Web application developers and testers to play with and should not probably be used by the great unwashed, who should stick to Firefox 1.x.
Firefox 2.0 will probably be out early next year and there will be a couple more betas around in Autumn and Christmas.
There are a few screen shots and more details here . µ