Thunderbird is Mozilla's mail/messaging application. These pages document Thunderbird and also provide links to documentation about the MailNews backend which is also used in other projects such as Eudora/Penelope, Seamonkey, Correo, etc.
Thunderbird is Firefox's kid sibling, and is built on the same technical platform as the web browser. In development for many years, and currently one of the most popular open source email clients, it is used by millions of people around the world to bring together all their email accounts, newsgroup and feed reading in a familiar high-productivity environment. (From early 2007 to early 2011 Thunderbird was developed by Mozilla Messaging, a subsidiary that was owned by Mozilla.)