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 to enable the option for that in Thunderbird's settings (Windows-only on Thunderbird 76+) or install an independent application to move Thunderbird to the tray whenever it is minimized. On Linux I'd recommend KDocker (available in many Linux distributions).