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.
https://www.thunderbird.net/en-US/thunderbird/60.0beta/releasenotes/
iCalendar appears to be available (read-only).
https://support.mozilla.org/en-US/kb/creating-new-calendars
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. 
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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.