Image may be NSFW.
Clik here to view.Reo-2007 has written a theme to give Thunderbird the “leopard Mail look”. This is the first Leopard Mail skin I’ve seen for Thunderbird, although others exist to give Mail.app the Tiger Mail and Panther Mail look.
If, for some reason, you have to use Thunderbird, it might as well look as much like Apple Mail as possible.
Reo-2007′s skin is a mixture of theme, extensions and customised CSS which combine to give the look of Leopard Mail as well as some of Leopard Mail’s new functionality, like Notes.
The overall effect is well done:
Clik here to view.

One extension provides the option to add Notes to emails, just like you can in Leopard Mail. When the Notes toolbar icon is clicked, a pop-up window appears:
Clik here to view.

The skin will only work on Thunderbird 2.0+. You can get it from Reo-2007′s page Image may be NSFW.
Clik here to view. on DeviantART.