Now that Aurora is dead I had to switch to Thunderbird Beta for testing. But when using an existing profile with Thunderbird Beta or when updating from one Beta build to the next no matching Lightning package will be installed. (Lightning installation happens only when creating a new profile as far as I know.) Currently I workaround by manually downloading a matching package from https://archive.mozilla.org/pub/calendar/lightning/candidates/ But this is not acceptable for most users. If you want to have good test coverage on Lightning Beta you need to release Lightning Beta builds on https://addons.mozilla.org/thunderbird/addon/lightning/versions/beta to ensure that they can be found by users and by Thunderbird internal update mechanism.
Hmm so my impression was that the matching Lightning packaged with Thunderbird is installed when a new version of Thunderbird is downloaded. If that is not the case we should definitely try to fix that. I can surely upload them to AMO as an intermediate workaround.