Rated 5 out of 5 stars

Works with TB 78.6.0 in both direction.
But I had to fiddle some hours in a TB version before (think, it was TB 78.4 or so), since then, it works.

As far as I remember, I had to cleanup TB cache

Then in a window in TB enter my Google credentials (and not click anything else while doing this, otherwise TB didn't remember my credentials correctly, maybe it forks to Firefox) and authorize the Provider as third party app (see https://support.google.com/accounts/answer/3466521?hl=en).

Rated 5 out of 5 stars

TB 78.4, had to update PfGC by xpi file. My calender disappeared, so I had to define it new. No new password entry needed, works !
Thanks !

Rated 5 out of 5 stars

TB 68.2.1 lightning with Google calendar - requirement in Nov 2019:
Open about:config in TB --> general.useragent.compatMode.firefox = true
This should be noted in the heading of this website to reduce the low ratings :-)

This review is for a previous version of the add-on (68.2.1).