Try this. Go to Computer > C: > Users > [your user name] > app data (you may have to change settings to show hidden files and folders) > Local. From there, look for something like Mozilla or Thunderbird. If you find Mozilla, then open that folder, then go to the Thunderbird folder. Copy everything within that folder to a usb drive. After you reformat and reinstall Thunderbird, go to that same folder. Replace everything in it, with what you copied from the old install. I did this with Google Chrome before and it worked perfectly.