Lightning is a project from the Mozilla Foundation originally designed as an extension ("add-on") that adds calendar and scheduling functionality to the Mozilla Thunderbird mail client and SeaMonkey internet suite.

[6] Sun Microsystems contributed significantly to the Lightning Project to provide users with an alternative free and open-source choice to Microsoft Office by combining OpenOffice.org and Thunderbird with the Lightning Extension.

[7] In addition to general bug-fixing, Sun focused on calendar views, team/collaboration features and support for the Sun Java System Calendar Server.

A calendar was originally to be fully integrated into Thunderbird 3, but those plans were changed due to concerns with the product's maturity and level of support.

[5][13] With the 2020 release of Thunderbird 78, Lightning is now a permanent part of the program.