Discontinued
Development on the Sunbird project has halted and its final release was on March 30, 2010.
Mozilla Sunbird was a cross-platform calendar application developed by the Mozilla Foundation, Sun Microsystems and many volunteers. Our goal was to provide you with full-featured and easy to use calendar application that you can use around the world. 31 languages; Addon for bidirectional access to 
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Reliable, simple, light. Still using it for years
This is my calendar application by choice. Indispensable. I use it every hour, every day. Could not cope without it. I don't care that it is 'discontinued'. Works excellent for me! :-)
Sadly, Sunbird has been discontinued in favour of the Thunderbird-integrated Lightning plug-in, for a lack of developers. 1.0 Beta 1 was the last release. People who prefer a stand-alone calendar application will have to look elsewhere.
used this until lighting was updated for Thunderbird. now that i have lighting there is no real need for sunbird.