rbtray will do this. It's small, free, and open source
https://www.google.com/search?q=rbtray
Just right click on the minimize button
Minimize to Tray Revived 1.1.2 works just fine for me as a Thunderbird 17 addin:
https://addons.mozilla.org/en-US/thunderbird/addon/minimizetotray-revived/?src=ss
Minimize to Tray Revived 1.1.2 works just fine for me as a Thunderbird 17 addin:
https://addons.mozilla.org/en-US/thunderbird/addon/minimizetotray-revived/?src=ss
Try filing a feature request. Ya never know, they might do it.