I have unwanted notification for events coming in the past. I don't succeed to cancel it. I have Windows 10, using Thunderbird 52.2. I tried also with 55.0b2, same issue. The event is created from a Lotus Notes calendar, and I am invited to such event, I deleted the event, the notification continues to appear. I tried to apply the solutions presented, but the issue is still there. So not read only and event notification unchecked. Attached, the screenshot of the notification. I guess that there should be a way to delete such notification.