Is this a snapshot of active profile or the prior profile? I see you also have a default.esr profile. Did you check to verify that Local Folders aren't hidden (see the three dots to right of 'new message' button). Another place to look is clicking the sbd entries as some local folders may be there. The msf entries can be ignored, as they are just indexes that thunderbird automatically reconstructs. If the folders are actually missing, you can coy them from wherever they are to the Mail\Local Folders folder when thunderbird isn't running and then restart.

It could be that Thunderbird is simply using the wrong profile and, even though the folders are there, it's looking for them in the other profile folder. You may be experiencing the issue described in the following article:

https://support.mozilla.org/kb/recover-user-data-missing-after-thunderbird-update

Thanks so much David and DavidGG

From following up on both your sets of comments I have discovered that I have two profiles folders, one that contains all the emails etc from before I upgraded, and the other my new emails etc. I have also discovered that I can make either the default profile and this will then determine what I see when I open TB -either the old or the new.

Now I have to check out how to merge them into one default folder, by copying from one into the other. Maybe all I need to do is to copy the contents of the MAIL folder from the old into the new and not all the other stuff?

Any way it's getting on towards midnight here in New Zealand so I'll resume the quest tomorrow.

You may do this by moving/copying the corresponding mbox files from the profile you don't want to use to some place within Local Folders inside the Mail folder of the profile you want to keep using. Thunderbird will automatically recognise those files and show them as folders at the corresponding location under Local Folders the next time it is launched without you having to do anything else.

Mail in account folders will be stored within a folder named after the incoming mail server, either in the Mail or the ImapMail folder within the profile folder, depending on whether the account is POP or IMAP.

You only have to move/copy the mbox files without filename extension. The .msf index files will be automatically created by Thunderbird again if not present.

Quit Thunderbird first, and make a backup of the enclosing Profiles folder before starting to move files around, just in case.

Thanks DavidGG. Finally achieved the solution!