https://support.mozilla.org/en-US/questions/1261974


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It is called caldav

CalDAV

    Choose CalDAV.
    Enter the URL in the Location field and click Next.
    Choose the name and color for the calendar you just added.
    If you also check the Offline Support option, you can use your calendar when you are offline.
    Click Next.
    You'll get a confirmation that your calendar has been created. Click Finish. 

To quote https://developers.google.com/calendar/caldav/v2/guide

The URL specified in 2 above is https://apidata.googleusercontent.com/caldav/v2/calid/events Where calid should be replaced by the "calendar ID" of the calendar to be accessed. This can be found through the Google Calendar web interface as follows: in the pull-down menu next to the calendar name, select Calendar Settings. On the resulting page the calendar ID is shown in a section labelled Calendar Address. The calendar ID for a user's primary calendar is the same as that user's email address.
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