I'm on Thunderbird 78.4.1 release. I can add an event in Thunderbird calendar, but it does not get added to Google calendar. I can add an event in Google calendar and it does get added to Thunderbird calendar. I can delete an event in Thunderbird calendar, that is also in Google calendar, and it does get deleted in Google calendar. I have 3 extensions: Provider for CalDav & CardDav, Provider for Google Calendar, & TbSync. I've tried enabling & disabling all combinations, but still cannot get Thunderbird calendar event to get added to Google calendar. Is there a post specifically for my release of Thunderbird that explains how to properly sync these calendars?