User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:91.0) Gecko/20100101 Firefox/91.0

Steps to reproduce:

OS: macOS Big Sur, Version 11.5.2
Thunderbird: Version 91.0.3 (64 bit)

1. Right click on the Virtual Folder (Unified View)
2. Select Properties
3. "Change" the folders in which should be searched
   ( 4) Check the box for an arbitrary folder )
4. Press Ok/Cancel

Actual results:

see image below;

the edit window gets hidden behind the Thunderbird window. However, you can drag the Thunderbird window around (as I did to create the screenshot).

Not any click target can be clicked in any of the two windows. However, the menu bar is still operative. In order to get rid of the view (in my experience) you have to close Thunderbird via the menu bar.

Additional Information:
When editing the "virtualFolders.dat" file in the Library/Thunderbird folder, it gets overwritten with the old data after closing thunderbird and changes get lost. However, in my case, only the changes to the "Sent" unified folder will not persist.

The others were saved.

Expected results:

The editing should be possible and the windows should still be operative.

I think this goes back to bug 1699514