As you may know, Firefox 3 introduces three different themes for all the officially supported operating systems: Firelight on Mac OS X, Tango on Linux, and Strata on Windows, which even comes in XP and Vista variations to better blend with Luna and Aero respectively.
If you are feeling envious of what the other platform or style got or just want to try a different look and have a couple of minutes to spare, you may want to give these themes a try.
Vista on XP, is a theme by Pascal Herbert that lets you use the Windows Vista theme, which is pretty much like the XP theme except all main navigation bar icons are blue and glassy instead of plastic-y.
If you prefer XP’s green plastic, try XP on Vista by the same author.
MacOS X Theme, created by GOLF-AT, brings Firelight’s brushed metal to users in Linux and Windows platforms. You will need a Mozilla Add-ons account though, as it is labeled experimental and awaiting further reviews.
Finally, if you don’t like change and believe Firefox 2 theme was just perfect, there’s also a theme for you: Firefox 2 for Firefox 3 will make you feel as it is 2006 again, yellow location bar for secure sites included.