How to set up private two-way sync between iCloud Calendars and Thunderbird v78

I do want to add that TBSync v2.16 is known to also work with setting up iCloud and Google calendars as well (for those needing Exchange ActiveSync, that one has been updated. You can download it here).

TBSync is very spotty with maintaining a solid connection to iCloud (in TB v78) and will not tell you or give you an error message to let you know it isn't syncing. But the devs for TBSync have a good reputation so I am sure that it will be updated soon. :) MHO

With TBSync currently you won't be able to change the names of the calendars as they are being directly synced with the account, lest you drop the connection to that account. [When TBSync pulls all all calendar info (and associated sub-calendars (by their names) from that account) it dumps them into a collective group in TB v78, like ({Google) <calendarname>).

So the above option will allow you to keep the original names without having the calendar names prefaced with the name of the account ("Google," "iCloud," or whatever the general account name is.