Do you mean the Lightning add-on[0] that is now bundled with Thunderbird [1]? I don't know if it's available as a stand-alone app. I use Thunderbird but not Lightning.

[0]https://addons.mozilla.org/thunderbird/addon/lightning/

[1]https://support.mozilla.org/t5/Calendar/Integration-into-Thu...

I use Radicale [0] CalDAV server and Lightning addon for Thunderbird - works great!

[0] http://radicale.org/