Rated 3 out of 5 stars

Something very odd is happening.
Recently, Google keeps signing this add-on out citing that it a suspicious app. I also tried using an app password instead of my actual Google passord and it refuses to accept it.

This review is for a previous version of the add-on (91.0.2). 

Rated 5 out of 5 stars

I found a workaround for now:

Delete your saved tokens via Options, Security, Saved Passwords. Delete all OAuth tokens for Google Calendar then closed TB.

Open TB. You will get a popup for each Google Account associated with the calendars you have. Pay close attention to the account name and authorise only the first pop-up, then CLOSE the rest, and close TB again. Repeat this process for each Google Account until all are authorised.

Hope that helps everyone

This review is for a previous version of the add-on (3.3).