It was developed as a standalone version of the Lightning calendar and scheduling extension for the Mozilla Thunderbird and SeaMonkey mail clients. Development of Sunbird was ended with release 1.0 beta 1 to focus on development of Mozilla Lightning. As of 2016, the “latest development version” of Sunbird was still 1.0b1 from January 2010, and no later version has been announced. Unlike Lightning, Sunbird does not receive updates to its time zones database anymore.
Sun Microsystems contributed significantly to the Lightning extension project to provide users with an alternative free and open source alternative to Microsoft Office by combining OpenOffice.org and Thunderbird/Lightning. Sun's key focus areas in addition to general bug fixing were calendar views, team/collaboration features and support for the Sun Java System Calendar Server. Since both projects share the same code base, any contribution to one of them is a direct contribution to the other.
Although it is released under a MPL, MPL/GPL/LGPL tri-license, there are trademark restrictions in place on Mozilla Sunbird which prevent the distribution of modified versions with the Mozilla branding.
As a result, the Debian project created Iceowl, a virtually identical version without the branding restrictions.