User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.12; rv:81.0) Gecko/20100101 Firefox/81.0

Steps to reproduce:

MacOS 10.12.6, 10.14.x and 10.15.7
Thunderbird 78.3.1

1. Launch Thunderbird
2. Close Thunderbird with the top-left red close button or from File -> Close menu item (app is still loaded but the main window is hidden)
3. Select "About Thunderbird", "Preferences", or "Quit Thunderbird" from the main Thunderbird application menu
4. These menu items do not work

This problem only occurs on Mac and only if Thunderbird is closed with the red button or from the File -> Close menu item.

Quitting the app is only possible from the context menu in the Dock.

Actual results:

Application menu items do not work.

Expected results:

Application menu items should work.

Confirmed with TB 78.3.1 on macOS 10.14.x.
Once triggered, these main application menu items still don't work after the main app window got restored via "Window > Mail & Newsgroups".

(In reply to Albert Scheiner [:alberts] from comment #1)

Confirmed with TB 78.3.1 on macOS 10.14.x.
Once triggered, these main application menu items still don't work after the main app window got restored via "Window > Mail & Newsgroups".

Also the case when the main app window is restored by clicking on the App in the Dock.