Rated 5 out of 5 stars

No longer needed by me, but very many thanks to the developer who has tried, and mostly succeeded, to keep track of changing applications and APIs for many years.
Hats off to Philipp!

This review is for a previous version of the add-on (115.0.2). 

Rated 1 out of 5 stars

As noted by others, unreliable with google calendar. No longer needed if you just want to sync events, as Thunderbird's "add remote calendar" works well and fast.

This review is for a previous version of the add-on (115.0.2).