Backup your entire Thunderbird profile. https://support.mozilla.org/kb/profiles-where-thunderbird-stores-user-data#w_backing-up-a-profile

I backed up the profile OK, but can't find a way to view the emails in there. None of the 'mail' files would open with any programme that I have.

You'd always open your messages with Thunderbird. In case of your profile getting corrupted you can restore the backed up profile.

I may not have understood what you're trying to achieve though.

Thanks christ1, What I want to do is to clear my IN and OUT boxes every few months, but I want to archive my emails elsewhere on my system, as I frequently need to refer to emails I received or sent a considerable time ago. When I was using LiveMail, I was able to archive messages using a csv format, and I don't seem to be able to do that with Thunderbird.

I don't see any point in storing individual messages outside of the Thunderbird profile. For read messages (either from Inbox or folder Read) I do use the built-in archive function. https://support.mozilla.org/kb/archived-messages

This moves messages to an Archive folder with subfolders per year.

For Sent messages I do the archiving manually to avoid mixing up received and sent messages. There are subfolders per year underneath the Sent folder.

For exporting messages you can take a look at this extension, but archiving messages that way sounds ridiculous to me. https://addons.mozilla.org/thunderbird/addon/importexporttools/

OK thanks. That's just what I want to do though. I'll find another way, daft though the idea might sound. Regards

Many thanks. That looks like the answer. Best Regards Peterphyll