Sunbird was once the stand-alone version of Lightning. The original strategy for Sunbird and Lightning was to have both an standalone product, as well as a plugin for Thunderbird.
That idea has been abandoned, and Sunbird is no longer maintained. For that reason we recommend people to go to Thunderbird / Lightning.
The original beta versions are still available on the mozilla website though.
Sunbird sends the following user agent:
Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.6; en-US; rv:22.214.171.124pre) Gecko/20091125 Calendar/1.0b1