I have this issue with my reminders for peoples' birthdays. I have an event on their birthday that comes from my Google contacts. That event has a reminder 1 week in advance. When the 1-week mark hits, I get an undismissable notification from Thunderbird. I can close the reminder window, but it'll show back up within a minute or two. I've tried removing and re-adding this calendar but it doesn't matter. When I look at the details for the event, I see that the Organizer field is Unknown Organizer <unknownorganizer@calendar.google.com>. My regular email address is listed as an attendee. This has been happening for at least the last several months, perhaps more than a year.