perhaps try forcing them to be rebuilt

On the toolbar > Help > Troubleshooting Information -> Show Folder button to display the profile in explorer or finder. Close Thunderbird

open the mail folder Delete the folder smart mailboxes in the mail folder. Restart Thunderbird.

That will force the recreation of the whole unified folder settings.

It actually corrected by itself when I restarted Windows, this after a Windows update. Restarting Thunderbird hadn't corrected it.

Since my update, Windows has restarted by itself, not following a Windows update. I don't know why - no observed fault.

Thunderbird's now gone back to how it was, as per the original question. When this happens, it isn't just the folder view that changes. My customised toolbar changes to what I assume is the default, with Today Pane (which I don't have) and status toolbar (which I don't have).

I don't want to customise the entire view again, if it'll revert to the default once again.

Thunderbird stores settings for the user interface in a file called session.json. Unfortunately ccLeaner for one deletes it to "protect your privacy" I doubt it is alone. Other hallmarks of the file being fiddled with are the reading pane reappearing after it is turned off. And open tabs being "lost". Optimisation tools of one sort or another are the usual cause of this as they delete the file which Mozilla Firefox only ever used to store the open tabs really and they incorrectly assume that the file is only a cache.

Hi, I tried the first method and also the session.json and neither worked. Still an empty Unified Folder View.