Mozilla Thunderbird is a free, cross-platform e-mail and news client developed by the Mozilla Foundation. The project strategy is modeled after Mozilla Firefox, a project aimed at creating a smaller and faster web browser. Just as Firefox aims to redefine the web browser, Thunderbird is a refinement of the mail and news interface. On December 7, 2004, version 1.0 was released, and received over 500,000 downloads in its first three days of release (and 1,000,000 in 10 days).

Thunderbird aims to be a simple e-mail, newsgroup and news feed client. It is not a personal information manager. Additional features, if needed, are often available via extensions.

Thunderbird can manage multiple e-mail and newsgroup accounts and supports multiple identities within accounts. Features like quick search, saved search folders ("virtual folders"), advanced message filtering, message grouping, and labels can help manage and find messages. On Linux-based systems, system mail (movemail) accounts are supported.

Thunderbird supports POP and IMAP. It also supports LDAP address completion. Both reading and writing of HTML e-mails are supported. The built-in RSS/Atom reader can also be used as a simple news aggregator.