I'm having the same issue and it's been driving me nuts all day. It appears https://apidata.googleusercontent.com/caldav/... that's linked to the Google calendar is not working and Thunderbird is not reading it. When I go to try and setup a new calendar linked to Google calendar, all the options are greyed out when it looks for the calendar on the Google network. Mozilla support, any insight on what's happening? Not a good way to start off the first Monday of the new year.