Thunderbird calendar needs to be able to sync Google calendar & iCalendar to be a universal programe without it it’s pointless using it as an everyday programe.
Missing Provider for Google Calendar is listed as the only unresolved Known Issue for Thunderbird 60.0b9.
iCalendar appears to be available (read-only).
Google calendar is working in Beta. There was a technical issue I personally experienced and worked with devs to resolve. I can now happily inform you that both iCAL and CalDAV are working.
The longer term Google Provider issue is more a discussion of what is to become core Thunderbird code, and what is deferred to an add-on. The main release channel version wil obviously not do without such an important feature. Beta channel is a test channel, where things do occasionally break.
It would be nice to have the Provider for Google Calendar add-on working in Thunderbird 60. The functionality it provides is essential.
I have tried above bit still does not sync as the issue with iCalander is you need a password entered that’s provided via icloud to allow permission to access iCalander etc. There is no provision for this at all so unresolved.
Same problem with Thunderbird 60.0. I have added two calendars from my Nextcloud via CalDAV. The calendars do not synchronize.