As there does not appear to be an answer to my problem, can anyone tell me how I can restore my calendar from a backup without also restoring the email messages (and so losing messages that have arrived since the problem happened)?

The calendar selection under View is always greyed out unless you have the calendar open and you are viewing the calendar tab. You open the calendar under Events and Tasks-Calendar. The shortcut is control+shift+c.

Thank you for that - I now have my calendar back, but it is coming up as a separate tab.

How can i restore it to the previous state where the calendar appeared in the main Firefox window to the right of the email messages?

Found it. It is confusingly under VIew/Show Today/Show Today pane

Confusing? View is where you modify the view of an existing window. If you want the today pane to appear in the main email window that is where you find that option.

Confusing is when you say you want a Thunderbird feature to appear in a Firefox window. :) Confusing is where your question seems to indicate that you have lost the entire Lightning calendar program and says nothing about just being able to see if in the today pane.

Glad you got it sorted out.

Yes, these interrelated programs can be easily confused.

What I found particularly confusing was calling a display that shows a calendar for a complete month, Events and Tasks, and completed tasks a "today" pane.

... and I HAD lost the complete Lightning display.