Mozilla Downloads is an archive of old projects such as the Grendel mail client and the Netscape Messaging Access SDK which use earlier versions of the mail/news code used by Thunderbird.
Postbox knowledge base. Postbox has a different GUI and features but has enough in common (since its based on Thunderbird) that some of the articles might be useful.
tb-enterprise is a mailing list to discuss issues about deploying and configuring Thunderbird in enterprise environments.
