Dear Mozilla Professional,
I would dearly love it if you would kindly allow me to add the ability to synchronize my iCalendar both read and write please? I have been successful in adding my iCalendar but it only works in read-only mode.
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Thunderbird has had caldav (read write) access to remote calendars for many many years first via the addon lightning and for the past few versions natively. As icloud supports caldav, I would suggest the capability already exists.
Perhaps you should ask in support about help in integrating your calendar in the existing read write formats.
From the official FAQ
always read-only; you won't be able to change anything for the calendars.