Follow these steps in order to install Enigmail in Thunderbird:
- Download Enigmail with Firefox using the context menu (right mouse click) -> Save As ...
- Open Thunderbird and go to Tools -> Extensions
- Click on the Install button and select Enigmail
- Restart Thunderbird
If you are looking for a GnuPG extension for Firefox (e.g. to read web mail), then check the FireGPG extension.