Good idea! Rated 4 out of 5 stars
I am a Linux user, and none of the minimize extensions I tried worked well, so Thunderbird is always open in a specific desktop. The problem is that I miss new mails... Well, not anymore with this extension! Great work!
One question, though: actually, the popup box isn't refreshed, meaning that if I receive a new mail while the popup is already showing, it will continue to show the "old" new mail, it would be great if all new mails were shown.