I have this issue, too. I see plenty of references to it around the 'net, but no real solutions. I had to resort to stop using Thunderbird's calendar and relying on Google's online calendar. It's disappointing because I would like to get reminders, but not if can't be snoozed and dismissed.

I still hope to find the cause and solution someday.

I've been having this problem for a week or several. I deleted the Google Calendar from TB and re-attached it using the Wiki instructions and so far it's working again.

Where can I find these Wiki instructions? I have in the past removed and re-added the calendar without good effect, but maybe I missed a step. Or maybe Google has made a fix in their calendar since I last tried.

In my case the problem turned out to be rooted in an event that was shared to my iPhone calendar. This event was fed via a shared calendar at Google, where someone else created the event. TB was not able to delete or change that event, so I kept getting the appointment reminders. I went into my iPhone calendar (which I don't use) and deleted the shared event, and the problem appears to be fixed.

Whether this applies to any event in an iPhone calendar or only those shared to it I don't know. Hope this helps.

Update: today I learned that iPhone's calendar pulls birthday data from contacts and automatically inserts that as an event. I was unable to delete the event from TB, but after I disabled Calendar on my phone was able to delete the event. Hope this helps.

I just removed all my Google calendars from Thunderbird and added them back. I notice now that I'm not offered a Contacts calendar, which had previously resulted in double entries for birthdays and anniversaries. Those entries still exist in the regular Google calendar, so I'm not missing anything. I have yet to be able to trigger a reminder, but instinct tells me the Contacts calendar was related to the duplicated events issue, and maybe/hopefully that and the unkillable reminders will be gone for good. I'm reading this as a Google problem that they fixed recently.

Fingers crossed...

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.

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.

I find complaints of this going back many years. I can't tell if every Google+Thunderbird Calendar user has the same problem, but many do. I find some suggestions, but no real explanations or solutions. I've resorted to turning off all calendar notifications in Thunderbird. Bummer, as I'd love to get reminders, but not these unusable ones. At this late date I doubt there will ever be a fix.