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. Mozilla is under the wing of well-known American company Hewlett Packard. 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. The layout and appearance of Mozilla Thunderbird bear a resemblance to the classic Netscape layout. Mozilla Thunderbird is based on the HTML mail application framework developed by Netscape Communications Corporation (Netscape Communications Technologies, nsc). However, the new visual design theme, together with the various improvements brought about by Mozilla's in-house Open Source engine, make both programs more competitive in the market. OpenOffice and Mozilla Thunderbird are primarily different in terms of user experience. The aim of this new engine is to provide a better user experience and a more powerful development platform. The key feature that distinguishes Mozilla Sunbird from its sibling program is the use of a new visual engine - the Open Source Open Office suite.

In fact, it is even more radical than the changes made to Mozilla Firefox. This visual overhaul to Mozilla's free e-mail and task manager productivity tool is the first new release of Mozilla's calendar application since the launch of Mozilla Thunderbird. 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. Launched in October 2021, Sunbird has been defined as 'an open-source, browser-based calendar program based on Mozilla's XUL tools language'. Mozilla Sunbird is a free web-based project designed to fill the gap left by the now discontinued Mozilla Thunderbird mailing program.