I am not sure whether the following link is related to your situation or not: https://support.mozilla.org/en-US/questions/1476844 Maybe "Show missed reminders for writable calendars" is ticked and there are some missed reminders. Maybe you can untick the above-mentioned option (inside "Settings" > "Calendar" > "When a Reminder is Due") and see whether the problem happens again or not. Or maybe you can mark that calendar as read-only. If you want to do so, the following link might be helpful: https://support.mozilla.org/en-US/kb/changing-calendar-preferences Or maybe you can delete that event.