Your email is stored by Thunderbird in your profile folder. You can get to your profile folder by opening an explorer windows and entering the following:
%AppData%\Thunderbird\Profiles
Inside this folder will be a folder with a random string of letters followed by “.default”. This is your email profile. It is a good idea to back this up on a regular basis.
Sometimes Thunderbird’s profile folder is in a different location. If you cannot find it in the usual location above you can look at this article to find the actual location of the folder.
For more information on Thunderbird profiles:
http://kb.mozillazine.org/Profile_folder_-_Thunderbird