Mozilla Sunbird is a free web-based project designed to fill the gap left by the now discontinued Mozilla Thunderbird mailing program. Launched in October 2021, Sunbird has been defined as "an open-source, browser-based calendar program based on Mozilla's XUL tools language". Announced in September 2021, Sunbird is a web-based, free-of-licensing, standalone version of the Mozilla calendar and task manager extension for the Mozilla Thunderbird mail client and the Mozilla SeaMobile browser.
Mozilla Thunderbird is based on the HTML mail application framework developed by Netscape Communications Corporation (Netscape Communications Technologies, nsc). The layout and appearance of Mozilla Thunderbird bear a resemblance to the classic Netscape layout. A number of prominent web designers, such as Rob Reis, have also contributed to the development of Mozilla Thunderbird, including content authoring specialist Christian Dalsgaard and web designer Michael Cadeoles. Mozilla is under the wing of well-known American company Hewlett Packard. As it is still in the process of transitioning to using its own codebase instead of those of other browser developers, Mozilla is not ready to ditch its codebase, which means that some former Mozilla contributors continue to work on the project full-time. There are, however, quite a few developers that are leaving Mozilla because of tension between Mozilla and Microsoft, and thus a potential collision between the two companies could occur in the near future.