From MozillaZine Knowledge Base

Thunderbird cannot minimize itself to the system tray. However, if you're running Microsoft Windows you could try one of these solutions:

• MinimizeToTray: an extension for Thunderbird, Firefox, and Sunbird. There is also a Minimize To Tray Enhancer extension that adds more features to it. See [1] and [2] for more information about each extension.
• Thunderbird-Tray.
• One of the general minimize-to-tray utilities mentioned in this MozillaZine Forums thread or below.

If you are using GNOME, KDE, XFCE, or Fluxbox you can use kdocker or AllTray.

Minimize-to-tray utilities

With the following utilities you can minimize not only Thunderbird, but any program to the system tray.

• trayit
• trayconizer
• rbtray
• tray commander lite

Email notification utilities

Some email notification utilities that support the system tray:

• Fastcheck (Fastmail)
• Gmail notifier
• Notifier2 (GMail, Hotmail, Yahoo, Ymail or Rocketmail)
• Pop Peeper
• Pop Tray

See also

• Disable the new mail system tray icon