A recent Thunderbird update on Debian/Ubuntu systems has resulted in the loss of the Calendar.
As it is the only calendar that I use, I'd be lost without it.
So what's the fix?
The calendar feature with the Thunderbird email application is an add-on/extension called Lightning. It has had a bit of a troubled past during the last 10 years, mainly following Thunderbird updates where Lightning has been deemed to be incompatible with the current Thunderbird version.
However, in recent times it has been as good as gold.
The Thunderbird transition from version 59.x to 60.x seems to have created a new problem. I see that several people are suggesting its time for the calendar functionality to be built into Thunderbird, especially as Lightning is believed to be its #1 add-on. But it seems unlikely that that will happen.
If you are on a Debian/Ubuntu based Linux distribution and an English language user, there is some good news. The fix is to simply remove the Lightning extension and then install it from the repository.
For example, type in a terminal:-
sudo apt install xul-ext-lightning
However, there seems to be a localisation problem. So if you need non-English language you may need to look for another solution.
Further reading: https://bugzilla.mozilla.org/show_bug.cgi?id=1484532