From what I remember, I deleted "tags.msf" to rebuild the virtual tags folders, but to my surprise that didn't repair the missing folders.

Any other ideas ?

I would try the method for resetting Unified folders:

https://support.mozilla.org/en-US/questions/1429758

@sfhowes : thanks. I deleted "mail\smart mailboxes" and "virtualFolders.dat". Result: When restarting TB, all virtual tags folders (VTF) appear. So I thought, this would have solved the problem ... But then, I restarted TB again and some VTF have disappeared. So basically the problem is "solved" for one session, but TB doesn't "remember" it.

1) tag count before "resetting Unified folders": 11 2) tag count after "resetting Unified folders" + restarting: 15 (100%) 3) tag count after restarting TB again: 11 4) tag count after restarting TB again: 11

What could be the culprit ?

Not sure of the cause, but when things like this change, my first idea is that there's an external app that acts on TB and resets settings between sessions. The classic example is CCleaner that is well known to do this unless TB is excluded. Make sure when you delete the smart mailboxes folder and the dat file that TB is closed.

Thanks, good idea.

I discovered the problem: TB doesn't like the fact that some of my tags start with the "!" character (in order to prioritize them).

Someone should file this as a bug. (Can do in the future, but am unable to currently.)

Someone should file this as a bug.

Please do.