Note: The following did not display correctly.

'******' = characters I think I should not be divulging

For every .msf file, you should see a file of the same name without an extension, and of significant size, which contain the messages. If they are missing, I suspect they have been quarantined by FSecure.

Helpful as I now know where the actual emails are but.....

In 2 of my paths the number of .msf files matches the number of files with no extension.

In the 3rd there are more .msf files than no extension files. Difference is 10. I presume Archive, Archive-1 (has this been deleted at some time or is it sub directory related), Drafts, Junk, Sent, Templates & Trash do not need a no extension file, that's 7 files different. There are 3 .msf files that I do not see any no extension files. These files (folders) lacking a no extension file are fully active in Thunderbird happily receiving mails as they come in.

FSecure indicates no quarantine files.

I also notice a couple of .msf files with greater size than the no extension files. Again, these folders are fully active in Thunderbird.

What may have cause the .msf files / no extension files mismatch? Is something corrupt?

I don't know why you have a mismatch - perhaps some folders were unsubscribed (IMAP account), leaving behind an orphaned msf file. If you delete an msf file, while TB is closed, it will be automatically recreated when TB is restarted. But if the related mbox file is missing, it probably won't reappear.

Just to be clear, as you mention mbox files which I do not find.

The no extension files are the equivalent of mbox files?

I am setup to see ll file extensions. See Image.

Yes, the mbox files have no extension. They can be viewed i a text editor to confirm they contain messages.

http://kb.mozillazine.org/Importing_folders#Find_your_old_profile

Many thanks for your help.