Upon upgrading from 51? to 52, calendar is nowhere available. If I download the linux .xpi (5.4b4) and attempt to install it, I get the message "Lightning could not be installed because it is not compatible with Thunderbird 52.0." corresponding to the error: 1492457864092 addons.xpi WARN Add-on {e2fda1a4-762b-4020-b5ad-a41df1933103} is not compatible with platform. The lightning extension used to work; what makes it only compatible with linux all of a sudden?
There has been no thunderbird 51. Lightning is being shipped with thunderbird for over 5 years now, there's no need to use the external plugin (besides, the plugin from AMO laggs behind and is linux only). Does this affect new profiles or old ones only?
I was attempting to add the plugin because it did not come up when I started tbird, and I was told on the mozilla list I could load it from the extension pkg. This is an old profile. My error on the reference to a version 51. Based on the backup package created by portmaster when I built version 52, the previous version I was running was 45.6.0_2. If I move my ~/.thunderbird directory aside and start tbird, it comes up with the calendar ok. I suspect what happened is I hit a key/button by mistake and deleted the lightning extension. I'm guessing that if one does that, there's no way to reactivate it without starting from a clean profile, since one can't install it from the downloadable extension. Does that make sense? So, thanks for the hint about new vs. existing profile. I fixed my problem by hand editing prefs.js to add the lightning extension to extensions.enabledAddons and extensions.xpiState from an empty tbird startup profile, and copying the lightning extension directory tree from the startup profile into my profile's extensions subdir. Don't know if that was the "right" way to do it or not, but it seems to have worked. However, I think an issue remains. If one deletes the lightning extension mistakenly, it seems like there should be a way to add it back in without having to start from scratch and copy over existing data.
After ports r470672 Linux version of Lightning from addons.mozilla.org should work fine as libical is part of Thunderbird when build with --enable-calendar. Not sure if this will continue to work after Thunderbird 60.
I close here with not a bug. Please, reopen if there still problems.