I try and check out all the themes for Firefox and Thunderbird that I can, however only one has ever really impressed me. That theme is Charamel.

Charamel Firefox and Thunderbird Theme

This theme has had a lot of work put into it and it’s absolutely beautiful. Everything works together and it’s a joy to use. There are even elements in Thunderbird that appear as if their overlapping other parts of the design and it’s great. Not only is this very sweet eye candy, it’s also been designed to work better than the original theme I believe.

If you are in the market for an excellent Firefox or Thunderbird theme, check out Charamel. By far, it’s the best theme I’ve found.

Note: I’m not sure what’s up with their webstie as it looks awful right now. But that shouldn’t stop you from trying it.
Download for PRE 1.5 Charamel for Firefox.
Download Charamel for Thunderbird.

1.5 Update
The themes seem to come and go. Here are two links that I found, but I’m not sure which is which or if they are just two versions of the Firefox theme. :

Keep an eye on the official Mozilla Update site as they do appear and disappear from there every now and then.