Assuming that you are connected to an IMAP server and running on Windows 7, the data is stored in:
Where "USER" is your user name and "yg4g294c" will be different on your system.
In that folder, you will find a bunch of stuff including sub-folders for each server you connect to. An in there are a sub folder for the top level of the mail server and in there are files that contain the actual mail.
It should kind of make sense when you look at it.
If you are using POP3 rather than IMAP, I think you should see similar things though with a slightly different layout under a POPMail folder - though I'm a little rusty on POP3 mail in Thunderbird as I haven't used it for a very long time.
Restoring though is going to be trickier if you are using POP3. Using IMAP is easy, delete the profile and start again. Though that does loose your filters, saved searches and so on so export what you want before deleting. With IMAP, everything is kept on the server and Thunderbird just keeps a local copy if you ask it to.
Let me know if you are using POP3 and find a file for the folder you want to restore and I'll try to remember how to restore it (if it is possible).