At the top right of the Thunderbird window, click the menu button , then select Help > Troubleshooting Information > Profile Folder > Open Folder.

This will open your Thunderbird profile folder in Windows Explorer. Please post the entire path to the profile including the name of the profile folder.

Is this what you mean?

C:\Users\Ian\AppData\Roaming\Thunderbird\Profiles\t6aywppr.default

Good, there's only one profile.

Before anything else, make sure you do have a full backup of your Thunderbird profile. http://kb.mozillazine.org/Thunderbird_:_FAQs_:_Backing_Up_and_Restoring

Then try to install this add-on and import the folders underneath t6aywppr.default\Mail\Local Folders, which are of about the right size. https://addons.mozilla.org/en-US/thunderbird/addon/importexporttools/

Thanks for that. I've installed the add-on and tried importing the folders, but nothing seems to happen.

The directory looks like this: >dir/t

Volume in drive C is OS
Volume Serial Number is 4E14-2DA4

Directory of C:\Users\Ian\AppData\Roaming\Thunderbird\Profiles\t6aywppr.default\Mail\Local Folders\Archives.sbd

21/11/2015 11:49 AM <DIR> . 21/11/2015 11:49 AM <DIR> .. 13/08/2015 02:38 PM 0 Ian 07/02/2016 02:12 PM 2,390 Ian.msf 13/08/2015 02:38 PM 0 KPMA ceo 07/02/2016 02:13 PM 2,082 KPMA ceo.msf 28/08/2015 03:43 PM <DIR> KPMA ceo.sbd 13/08/2015 02:38 PM 0 KPMA Chair 07/02/2016 03:30 PM 2,737 KPMA Chair.msf 02/02/2016 06:34 PM <DIR> KPMA Chair.sbd

              6 File(s)          7,209 bytes

From the Thunderbird documentation I would expect the mail to be in the folders without extensions, but these are empty. Does this mean the files are corrupted?

From the Thunderbird documentation I would expect the mail to be in the folders without extensions

Correct.

but these are empty.

I thought you mentioned something about files of about the right size, not being recognized by Thunderbird. With a size of 0 bytes there obviously is nothing to be recovered.

Do you have a backup?

You may want to take a look at this article, but don't hold your breath to recover any messages. http://kb.mozillazine.org/Disappearing_mail

Corruption is often caused by anti-virus software messing with Thunderbird mail files. It is therefore recommended to create an exception for the Thunderbird profile folder, so that the real-time scanner won't attempt to scan the profile with your mail.

Thanks. That's what I'd come to expect.

The files that are "about the right size" are the ones with extension .msf which contain what looks like the text of emails surrounded by what looks like some sort of markup language. When I originally posted, I hadn't read far enough into the documentation to know that the extensionless files hold the mail.

Is there any hope of retrieving the messages from the .msf files?

No, .msf files are just index files.

OK. Thanks for your help.