When I receive an invitation from outlook, with a multipart message with Content-Type: text/calendar the event is rendered properly, but I am getting a "No writable calendars are configured for invitations..." error message. I have a google agenda set up (googleapi://...), and I can see and create events in it, two way (at google calendar and thunderbird), but the message remains. Creating a local a agenda fix the error, but doesn't sync with google. Checking the issue https://support.mozilla.org/en-US/questions/1097284 (and https://support.mozilla.org/en-US/questions/1108836) I went for creating a new agenda using iCAL- and the result is a worse error popup: MODIFICATION_FAILED I then used the https://www.google.com/calendar/dav/<emailaddress>/events option for calDav, it prompts for login forever. Then, searching even more I found another '''URL for calDav: https://apidata.googleusercontent.com/caldav/v2/your@email/events''' That finally worked. So, why don't you remove the "google calendar" option in the new agenda wizard if it doesn't work? Same for iCAL+google.