Thunderbird's reminders have been broken for 2-3 versions now. So what I do instead of clicking Dismiss(which is broken) is to drag them away and pile them offscreen so they stop bothering me.

You'd think mail clients would be a solved problem by now.

@neauoire using google calendar? had that weird bug, installing ‘Provider for Google Calendar’ helped
@aeweyrose The reminders are from events I accepted from emails I've received, but I don't think it's bound to google calendar, I'll have a look. Thanks for the tip