Use subversion repository
You can download the source code with Subversion:
svn co http://svn.getfiregpg.org/firegpg
You can use The Websvn too.
The source code is packaged as an archive too (but only with essential files):http://getfiregpg.org/stable/firegpg-0.8.tar.gz (Sign)
The source code is documented, you can have a look here: FireGPG's code documentation
For people who make packages for linux, or simply if you need it, you can use the url http://getfiregpg.org/stable/firegpg-version.xpi (or tar.gz) to get a specific version.
Versions prior to 0.6 are affected by security issues. DO NOT USE THEM IN A PRODUCTION ENVIRONEMENT !
Files are signed with the key of Maximilien Cuony (F1530A35)