Alex Faaborg has some mock-up images on his blog of what Firefox 3 will look like on both Microsoft's Windows Vista and Apple's Mac OS X. I have yet to see any images for what Firefox 3 may look on a Linux box and many of those commenting are also not happy Linux was left out of the picture.
You can see the mock-ups yourself by clicking here.
The visual for Firefox 3 on Vista looks very nice but I'm not so sure about the the visual for the Mac. The Mozilla bloggers are giving a lot of reasons for why Firefox on the Mac will look the way it does...and that worries me. The more effort someone spends trying to convince me of something, the more evident it is to me that the idea isn't selling itself.