About this Add-on
Closing the main window will still be permitted in either of the following cases:
- There is another open main window. This permits to close duplicate main windows, for example after opening a folder in a new window.
- The window is already minimized. This permits you to close Thunderbird without accessing Thunderbird's menu: close the window once (to minimize), then right-click it in the task bar and close it again.
If you want Thunderbird to minimize to the tray, you need an independent application to move it to the tray whenever it is minimized. On Linux I'd recommend KDocker (available in many Linux distributions), on Windows you can probably use TBTray (but I have not tested that).