Thunderbird's mailbox files don't have extensions. So you're looking for Inbox, Sent, Trash etc.

Another way to go about it would be to search for all the files with .msf extensions, create a list of these (dir *.msf > files.txt), strip off the .msf and what you'll be left with is a list of your mbox files. I'll leave it to you to work out how to traverse your nested folders structure; I'm on Linux here and can't experiment with MSDOS-style command lines.