I found some hints:
https://wiki.mozilla.org/Thunderbird:Pluggable_Mail_Stores#Current_Mail_Store
Currently, local mailboxes and offline news& IMAP stores use the berkeley mailbox format, a flat file in the Mbox family
and
http://whatis.techtarget.com/fileformat/MBOX-Mailbox-file-Berkeley-Unix
MBOX File Format
Mailbox file (Berkeley Unix)
MBOX is a file extension for a text file used to organize and store e-mail messages. MBOX stands for MailBOX.
The MBOX file is the most common format for storing email messages on a hard drive. All the messages for each mailbox are stored as a single, long, text file in a string of concatenated e-mail messages, starting with the “From” header of the message. MBOX files were used predominantly on Unix. MBOX files are still used with several current e-mail clients, including Mozilla Thunderbird, Microsoft Entourage and Qualcomm Eudora.
If you look in
preference -> advanced -> general -> config editor and search for msgstore you find
mail.serverDefaultStoreContractID = "@mozilla.org/msgstore/berekeleystore;1″