[4] Announced in July 2003,[5] Sunbird was a standalone version of the Mozilla Calendar Project.

It was developed as a standalone version of the Lightning calendar and scheduling extension for the Mozilla Thunderbird and SeaMonkey mail clients.

Unlike Lightning, Sunbird no longer receives updates to its time zone database.

Sun Microsystems contributed significantly to the Lightning extension project to provide users with a free and open-source alternative to Microsoft Office by combining OpenOffice.org and Thunderbird/Lightning.

As a result, the Debian project created Iceowl, a virtually identical version without the branding restrictions.