Mozilla Thunderbird to tray ...

Uhm, hi there,

could anyone please tell me if there is an option within Thunderbird, which allows the user to minimize it to the tray, that I just didn't noticed yet?

no, there isn't.

If you have $50usd, you can buy Stardock's Windows Blinds. Not only can you modify the shell appearance, but you can minimize to tray if you just set a simple user override. Enjoy.