Rated 5 out of 5 stars

I edited the .xpi for you and put it up on my Rapidshare account. I did the same thing a while ago for Fasterfox and it's still up also. Download the .xpi to your desktop, open Thunderbird 3.0, go to Tools ---> addons and drag the .xpi into the box. Works perfectly! (unless they go higher than version 5.0 anytime soon LOL)
http://rapidshare.com/files/326688439/MinimizeToTray-5.0.xpi