Thunderbird-Tray is a system tray launcher tool for Mozilla Thunderbird.
It hides Thunderbird when you minimize the Thunderbird window or when you click the tray icon.
Clicking the icon again will re-show the main window. Also it is possible to start Thunderbird by clicking on the tray icon. You can right click the Thunderbird-Tray icon to see a menu with the available options at any time. Thunderbird-Tray is open-source and licensed under the GPL.