Well, that's my story, and I'm sticking to it. Just a hunch that this is all working correctly in v.78. But Calendar does not need to configure anything in Tasks. The Tasks tool has options that also apply to it's access to Calendar, so both are available in my View menu when Tasks are open. I think the View menu is only a means to change some of the options for each tool, so it is only active when the appropriate tool is active. Both Calendar and Tasks return after restarting Thunderbird. the options in that menu are a checkbox that stays selected until you uncheck it. I think that the 'Events and Tasks' menu is the place to load these tools. But different is that with Tasks open, now both Calendar and Tasks are available in the View menu. Like before, with both of these tools closed, both items are grayed out in the View menu, and with Calendar opened, it is available in the View menu.

and the View menu now contains both Calendar and Tasks, and the 'Events and Tasks' menu has reappeared. In the new Thunderbird, v.78, I 'enabled' the default calendar (which I had not done before because I don't really use this copy on a VM).

Thunderbird tasks