Make sure your calendar isn't write-protected. Right-click the calendar in the left pane - Properties

Thanks for the suggestion, christ1.

I checked properties. There is check box labeled "Read Only" that was not checked. I assume that means the calendar is not write protected.

Immediately below that is a check box labeled "Show Reminders". It was checked, and I unchecked it. I'll see if that solves the problem.

I don't need to receive reminders from the Lightning calendar because I have another calendar app that I use to keep track of all appointments, etc.

It looks like all I had to do was turn off "Show Reminders", and those pesky notifications seem to have gone away.

This solves the problem for me, because I don't need meeting reminders from Lightning. (However, this wouldn't be a solution for somebody who only uses Lightning for calendar management.)

Thanks for the suggestion that led me to a solution!

I have similar issue, I get notification for past events, even if I delete them. The event is created from a Lotus Notes calendar, and I am invited to such event.

I tried to apply the solutions presented, but the issue is still there.

YReginster,

please start a new topic for your problem. Provide as many details as possible. https://support.mozilla.org/en-US/questions/new