I’ve found two themes for firefox and thunderbird that I’m so pleased with them I have to promote them a bit :-). They are Charamel and Silvermel created by Kurt Freudenthal. I discovered them thanks to Chewie. So, what I like about them is:
Works with newer versions of Mozilla firefox and thunderbird (Including version 3 beta 4)
It’s got really nice colours and a nice layout.
It works fine under linux and Mac OS X. In the latter it lets you have small icons in the bookmark bar, whereas the default theme does not allow you to do that. See image to see what I mean.
You’ve got two different colours to choose from.
So go and buy the guy a beer so he can work more on them :-).