Enigmail is a free and open source security extension to Mozilla mail clients such as Thunderbird and SeaMonkey. It allows you to write and receive email messages digitally signed and/or encrypted with the OpenPGP standard, in a simple and easy way. To download Enigmail, please use the "Download" menu on top of this page.
The Enigmail Handbook is a 123-page comprehensive document integrating detailed installation and usage information, technical references, FAQs, troubleshooting, and general tips and tricks.
You can view the Handbook online (below), or download the original PDF:
The current edition is aimed at Enigmail version 1.9 but contains also guidance on previous versions.
Setting up Enigmail