Review for Provider for Google Calendar by Eric Jul
Rated 2 out of 5 stars
Used it on MacOS Monterey 12,5.
Unfortunately, it appears to have a memory leak - after 24-48 hours, Thunderbird went from 1.5 Gbytes to 30 G :-(
And, quite often, there was a thread eating up 100% CPU time on a core, making Thunderbird slow and not very responsive. Uninstalled the add-on - and, voila, now my Thunderbird is 1,2 Gbytee pretty stable for many days, and quite responsive. And the calendar stuff works withou Provider - and I do not need the task stuff.
To create your own collections, you must have a Mozilla Add-ons account.