Comment 33 for bug 1797945

It was broken for me also and comments from #3 and #4 helped. Calendar is very important functionality and sudden disappearance was very problematic. May be work with Mozilla and ask them to update documentation for Ubuntu so that others facing this problem will have a quick solution.

I certainly did not know about the lightning package and always have installed from thunderbird. Another alternative could be update manager to warn people that it can break functionality.