[stomachmonkey]

Have not used Thunderbird in a while.

There are basically two ways that mail is stored, flat file or database.

Flat file means every email is an individual file on your hard drive, DB means they are all stored in one file that can get huge and if it gets corrupt can be a ***** to recover.

Most mail apps support mBox format.

What I would try is this, click hold and drag an email from TB to your desktop, if it makes a file you are good. Also add a folder in TB, likely now you have Inbox, Outbox, Sent, Junk etc...

Add one as you would to keep mail organized, put a couple of pieces of mail in it. Drag folder to desktop. Should give you an mBox.

Mboxs can be imported to a variety of different mail apps.