Rated 5 out of 5 stars
A lifesaver for me, now that Provider doesn't work for Google Calendars in v78.4! This works great for me.
I had to reset cookies as per this https://support.cdn.mozilla.net/hi-IN/questions/1310945 after "upgrading" to 78.4, in order to get my email to work again. So maybe doing that will get Open Google Calendar to work properly too. No guarantees but it's worth a try.