I have created the default icons for the Mozilla Thunderbird e-mail, feed and newsgroup client. (Not on the Mac version.)
The coming Thunderbird 2.0 will receive a major update to the theme. Below are pictures of it. It may change a bit before the final release.
The smiley images are created by Stephen Horlander. The application icon/logo is created by Jon Hicks.
I provided the default icons for the early Firefox versions 0.6 to 0.8, which is what led me to making the Thunderbird artwork. A further developed version of this Firefox theme is now available as Qute.