I also had a look at these posts: https://support.mozilla.org/en-US/questions/1192643

Unfortunately, Lightning doesn't show up in the list of extensions, nor does it show up when I search for it. Not sure though if this is still valid, as the post is 6 years old.

Hmmm, I now created a calendar via "new" --> Calendar to test, and somehow I managed to create a calendar, and then it gave me back the once grayed out "calendar" in view. "tasks" where still grayed out though. But stil, I couldn't view the calendar. But then i clicked on the top left corner symbol and I could activate the calendar tab somehow.

Not sure what is going on there, but this was annoying as hell and somehow I got lucky and got it back. Not intuitive at all. Can't remember how it worked before though. Well, seems like solved. Hopefully it works now.

Lightning has not been a part of Thunderbird calendar for years.

My guess is you use the provider for google calendar addon to get your google calendar and tasks and it is not present on your system or is disabled hence you new caldav calendar works but tasks do not as they are not part of the caldav spec..