As of July 2012, Thunderbird has transitioned to a new governance model, with new features being developed by the broader free software and open source community, and security fixes and improvements handled by Mozilla.
Thunderbird is a free, open-source, cross-platform e-mail and news (NNTP) client developed by the Mozilla Foundation. The project strategy is modeled after Mozilla Firefox, a project aimed at creating a Web browser.
Updated on Jul 31 2022
Full list of changes available here.