I'm almost sure this worked pre-102 but no more. When an event is due, the calendar reminder box pops up with the event which you can dismiss/snooze/etc. After actioning all events, the reminder box stays present. With the last event dismissed or snoozed, the reminder box should disappear automatically without having to close it manually.
Running Linux Mint Thunderbird updated yesterday to versio 102.2.2 and the Calendar Reminder dialog box does not close after using Dismiss All or Snooze All. As a matter of fact each reminder must be cleared with individual selects of Dismiss All or Snooze All