Make sure to select (highlight) a writeable calendar in the left pane when trying to paste. Although you can highlight any calendar, have multiple types enabled (checked), and copy events from any of them, apparently pasting only works when a writeable one is highlighted/focused. This may be [https://bugzilla.mozilla.org/show_bug.cgi?id=1203909 bug 1203909] - reports say the calendar to paste in needs to be selected, but I managed to paste in another one, at least an internal calendar - I haven’t tried a writeable .ics file. However, it makes sense to select a specific calendar first, since Lightning/Thunderbird has to know what calendar to store the event into. Can you please report back if this helps, and in a way to confirm the bug in case you think something is really wrong?