Just a quick update, it is still happening in version 91.2.0. Any time I try to add a folder to a saved search, it gets locked up in this really odd state where the "Edit Saved Search Properties" dialog stays up and locked behind the main Thunderbird window, and nothing works.

... another data point, even if I don't change anything on the folder selection, just opening and closing the dialog gets it stuck for me.

If anyone is in my situation and wants to try to fix it the hard way, you can manually edit the saved folder selection in ~/Library/Thunderbird/Profiles/[profile]/virtualFolders.dat - the "scope" value lists the folders with a pipe delimiter to separate them.

Or, you can delete virtualFolders.dat and Mail/smart mailboxes from the profile folder, with TB closed, then restart TB and set up the saved search again.

http://kb.mozillazine.org/Unified_Folders_not_working

Thanks @sfhowes. I tried that but the dialog still gets locked up. The smart folders work correctly if I edit the virtualFolders.dat file directly, but any time I try to click on the "Choose..." button to select folders to search and then close the dialog the interface gets locked up.

Does the dialog still lock up in safe/troubleshoot mode (hold Option when launching TB).

Yes, just tried it in safe mode and it behaves the same way.

It looks like this bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1699514

Editing the dat file manually appears to be an effective workaround.

Thanks @sfhowes - yes, that definitely sounds like it's describing this issue. I'll follow that bug and hopefully it will get resolved (crossing fingers, looks like it's already 7 months old...)