Thunderbird cache

Thunderbird cache : /mnt/data/pdat/mail/Thunderbird/445c0ssf.default/cache2/

The cache can be cleared using Tools/Options/Advanced/Network & Disk Space,

Thunderbird’s cache is used, for the most part, to store images
The files with the extension .msf are NOT mail files . The .msf files are used by Thunderbird to store user selected settings like the sort order for messages in the list display, tags assigned to messages, etc.

mail messages are stored in mbox files which will have the names of the Thunderbird folders (Inbox, Sent, etc.) with no file name extension.

http://kb.mozillazine.org/Files_and_folders_in_the_profile_-_Thunderbird