While we’re waiting for Firefox 4.0, which will bring significantly better compatibility with Windows 7, the Strata40 theme combined with StrataBuddy will make your Firefox 3.5/3.6 look pretty with Aero. It is the best theme available for the current version of Firefox in my opinion. StrataBuddy lets you customize the theme to your liking. Check out the screenshot below. Links are posted below the image.
P.S. I didn’t mention Vista because Vista is CRAP.