As of version 78.0, (first offered to channel users on July 17, 2020) the previously known addon called Lightning Calendar was fully integrated into Thunderbird. It would have required a huge rewrite in order to keep up with the massive changes going on and the number of problems was growing. There was a really large number of people asking for proper integration. It seemed the right time to go down that route as it was going to be difficult maintaining the addon by the author. It is no longer an addon or extension and cannot be removed. There have been many improvements and additional functionalities and since version 91.0, it default to CalDAV when supported by the server. This makes it so much easier and automatic when hooking up to eg: Gmail Calendar. It is under constant review and refinement. Recently, the devs modified code so that everytime you wanted to edit an event a double click did not open the edit facility - many people objected and now a preference has been created so people have the choice again.