If you have more than one email account and have set View>Folders>Unified, then TB makes their common folders, such as inbox, appear as one. These are virtual folders. To backup one account, click the virtual folder and it will open up to the individual folders, which can be backed up.

Thank you for your lightning response David. To help clarity, I have attached a 'clip' of the upper end of my folder set - also confirming that the default (apparently) setting 'view>folders>unified' applies here and that I also have two separate accounts with the same provider. But they have always had discrete Inboxes. The first of which is EMAP and the second POP. So I am hoping to understand which folders are 'linked or virtual' as you surmise.

I did, however, make one particular 'modification of convenience' that seemed to have been allowed for some time now, whereby I tried to manage one over-populated Inbox, by dragging some messages into two new 'overflow' folders (e.g. 'Geoff' & 'Which?') - which were initially attached to the Inbox concerned. See diagram.

Thinking that this was the reason for a non-responsive ImportExportTools, I moved the folders to the alternative position you can now see in 'Local Folders' and tried again. This time, the options 'Export folders with sub-folders (with structure)' and 'Export folders with sub-folders (simple)' are rendered unavailable. But now no Error message interestingly.

However, 'Export folder' and 'Export folder (zipped)' are still offered for individual folders - not a complete folder-set (?). All I was trying to do was to export my complete (structured) message history to a - 'zipped or other'- folder for security. Is there an alternate way to achieve this?

I suggest that, to export all, considering that you now have them in multiple folders, to use the export all messages in the folder. Do that for each desired folder and you will then have .eml files for each message that can be immediately read. I would then zip the folder. If you later want to import the messages, the same addon can do that.

Thanks. Sensible solution. Basic, safe, but labour intensive I suppose? I was visualising exporting all in one action to (say) MBOX - as if for transferring all messages into a different installation - but in this case for restoring to my existing setup if ever the folders corrupted. Any logic in this?

To restore to your current setup, I always recommend backing up entire profile. For individual folders, I recommend individual backups, whether zipped mbox layout or zipped .eml layout. It's a tossup. My suggestion was only if you were just wanting to archive the messages for later review, not to do a restore.

Long time (for various reasons) since I last discussed this, but the background reason for my question is because I have been wrestling with 'Connection to the Server' problems and also 'Message delivered but copy not directed to the Sent box' problems. And now wonder if the root of these 'faults' now retained within the Profile?

See my other 'Q:' 'Thunderbird Ver 91.4 failing to download mail' https://support.mozilla.org/en-US/questions/1362931

Having followed that (and other) advice without resolution, I 'Deleted & Reinstalled' T-bird have not cleared the problems; perhaps because they are embedded in my Profile (which I retained).

I now wonder if I might backup ONLY the Messages and Folders i.e. the 'Virtual' (one account is IMAP; the other POP3) and 'Local' (my off-line Archives), retaining the original structure in the event of a Restore.

This might allow me to 'try again' with a clean install plus a 'first-time' Import of my backed-up Messages & Folders. But I'm not sure how - nor with what addon or format. What do you think?