tested with: thunderbird 68.6.0 (64-bit) on GNU Linux Debian 10

if one uses the build in calendar of thunderbird (it was called sunbird, was replaced by lightning)

go into advanced config editor: (about:config)

type:

calendar.alarms.showmissed

and double-click on the entry

to change it to false

https://github.com/Ericsson/exchangecalendar/issues/546

then simply close thunderbird and restart it

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