I wish Mozilla Thunderbird could sit in the Windows application tray instead of taking space in the Task Bar. The email client is not like the other apps on my system, including the browser. It's running all the time when I am at work. There could be an option "Minimize to Application Tray" in Windows builds; with that option on, closing or minimizing Thunderbird's window would leave the app running, but only in the App Tray. Of course, the menu entry Quit will still really quit.