Rated 5 out of 5 stars

That "Download upgrade for Firefox 5.0" is actually:
https://addons.mozilla.org/en-US/thunderbird/addon/minimizetotray-revived/

It lacks features that this addon has. Which is a shame.

As for this addon until I can find a minimize to tray addon that works in TB5 and also minimizes on startup I'm not upgrading TB >.<