OK, after finding me a tray icon plugin for the Linux version of Thunderbird, I came to the conclusion that the default icon Debian is shipping with it [the non-branded Mozilla icon], is just ugly. It's a aliased icon [read: jagged edges etc], brown, and doesn't play nice with my Tango icon theme. So I looked where the default icons are stored and simply replaced them by stock Tango icons :)
How? Go to /usr/lib/thunderbird/chrome/icons/default, select some [Tango] icons you want to use as replacement, save them as .xpm files and overwrite the ones in that directory. You can download my Thunderbird Tango icons here.
Additionally, a complete Tango icon theme for Firefox can be downloaded here, and one for Thunderbird can be found there too.
A screenshot says a lot too:
I also put a Tango theme for Gaim 2.0 online. Enjoy :)
Clarified some text and fixed the uri pointing to the Thunderbird tray icon. Sorry :)