Go to "Edit | Mail Account Settings". From there, click on "Server Settings" for each of your accounts.
The bottom-right side of the screen should show a path to a file. (I'm not at my machine right now, and the exact format escapes me at the moment.
) That is the location of your mailbox.
Back up that file, and you will back up your Mozilla Mail.
I moved mine to another drive when I migrated from Windows to Fedora. All I had to do then was re-point the file to the old inbox -- and voila! Everything was there.
Adios,
Clay (Linux Newbie, but longtime Mozilla fan!)