The 'Event' (s) that you cannot dismiss etc. Can you offer a bit more information to see if there is something specific about those events you cannot dismiss etc.

Are all of them on gmail calendar or some in Home and some in Gmail ? Are they all events created as a result of an invite to an event? Or are they a mix eg: some events you created and some events created as a result of accepted invite ?

Do you find some events can be dismissed etc ? Do you find that once one event cannot be dismissed then this is now effecting all events? Is there anything those events have in common? Are they all set up as repeating events or are they set up as a 'Does not repeat' ? Are they all set up using a repeat 'Custom' option ?

When you next see a pop up alert, please make a note of the 'oldest' alert in list that did not dismiss.

Open the event for that oldest alert that will not stop sending you alerts change the reminder to 'No reminder'. 'Save and Close' event then make sure the gmail calendar synchronises.

The next time the alert pops up: Is that old event which you manually changed to no reminder, still in that alert reminder list? If no, try to dismiss etc the oldest alert displaying.

Please confirm this problem is only about 'Events' and not 'Tasks'. I'm needing feedback on this to see if: one failure to dismiss is causing a log jam on all others. the events have a common setup or setting

That a LOT of questions. I will do my best to answer them in order.

1. 1: Are all of them on gmail calendar or some in Home and some in Gmail ?

[A] All are on my Gmail calendar. I do not use my "home" option at all.

1. 2 Are they all events created as a result of an invite to an event?

[A] For the one's that are currently "hanging", none are the result of an invite. All were created by me in my calendar app within Thunderbird.

1. 3 Or are they a mix eg: some events you created and some events created as a result of accepted invite ?

[A] See answer above.

1. 4 Do you find some events can be dismissed etc ?

[A] Nope, once they start hanging.. none can be dismissed individually or in bulk (Dismiss all)

1. 5 Do you find that once one event cannot be dismissed then this is now effecting all events?

[A] This appears to be the case, because as I progress through the month, more and more are being added to the list that can't be dismissed.

1. 6 Is there anything those events have in common?

[A] Other than being created within Thunderbird, by me, for the same calendar (yes, I have multiple calendars but it's only my primary work calendar where this is happening)

1. 7 Are they all set up as repeating events or are they set up as a 'Does not repeat' ?

[A] Some are repeating (every Monday for example) and some are not.

1. 8 Are they all set up using a repeat 'Custom' option ?

[A] One is (not the oldest one) and the rest are either weekly or one-offs

1. 9 Your instructions "When you next see a pop up alert, please make a note of the 'oldest' alert in list that did not dismiss. Open the event..."

[A] I will try this. I've been turning off the notifications manually to get them to stop. It's annoying.

One final note... none of these are "tasks", all are "events".