Recently updated to check the beta version of Thunderbird (Version 43 ? ) and lightning is no longer available. Is there a version coming to support that?
How are you getting the TB Beta? It should be version 60.
Current ‘release’ channel is on v52. Lightning is now fully integrated in Thunderbird Beta (v60). This means you no longer need a separate add-on to fulfil your calendar needs. 60 is also due to roll to the release channel in a matter of days.