Mozilla Thunderbird is free and open-source email client software which also functions as a full personal information manager with a calendar and contactbook, as well as an RSS feed reader, chat client (IRC/XMPP/Matrix), and news client. Available cross-platform, it is operated by the Mozilla Foundation's subsidiary MZLA Technologies Corporation. Thunderbird is an independent, community-driven project that is managed and overseen by the Thunderbird Council, which is elected by the Thunderbird Community. The project strategy was originally modeled after that of Mozilla's Firefox Web browser and is an interface built on top of that Web browser.

The Mozilla Foundation is an American non-profit organization that exists to support and collectively lead the open source Mozilla project. Founded in July 2003, the organization sets the policies that govern development, operates key infrastructure and controls Mozilla trademarks and copyrights. It owns two taxable subsidiaries: the Mozilla Corporation, which employs many Mozilla developers and coordinates releases of the Mozilla Firefox web browser, and MZLA Technologies Corporation, which employs developers to work on the Mozilla Thunderbird email client and coordinate its releases. The Mozilla Foundation was founded by the Netscape-affiliated Mozilla Organization. The organization is currently based in the Silicon Valley city of Mountain View, California, United States.