Lightbird is a SeaMonkey extension that aims to provide a conventional interface for Lightning calendar. It is based on Sunbird code, and requires Lightning to work. Since it's just a frontend, it doesn't contain any binary components.

AMO listing: https://addons.mozilla.org/en-US/seamonkey/addon/lightbird/

MozillaZine thread: http://forums.mozillazine.org/viewtopic.php?f=48&t=2774051