The calendar won't launch at all now.
When opening Thunderbird, the Calendar tab used to be launched as well. Not anymore. When clicking on Calendar in the drop down menus, nothing happens at all and Thunderbird freezes up for a minute or so. Closing TBird and restart as no effect. This has been before (months ago), I could launch the Calendar and it would share tabs with TBird. Curiously, after restarting TBird, a Calendar window popups showing past scheduled events.
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Which drop down menu are you clicking Calendar? Under Events and Tasks is where it opens the calendar. If you are using the View menu you are in the wrong place. That is for setting options when you have the calendar open.
If you click the 3 bar menu button and select Add-ons do you see Lightning listed under Extensions? If yes, is it disabled? Hover over the word Lightning and see what version you have installed. What version of Thunderbird are you running. Look under Help-About Thunderbird.
If no, type Lightning in the search box on the add-on page and install it.