Minimize to system tray (Thunderbird)
From MozillaZine Knowledge Base
Thunderbird cannot minimise itself to the system tray, however for the Microsoft Windows version there is a utility called Thundertray which provides this function.
Download Thundertray and follow the instructions within.
If you use GNOME, you can (like with any mail program, or even without one) add the Inbox Monitor applet to your panel, and configure it to launch Thunderbird; however, each monitor can only watch one email account.
