The thread with my solution is at http://forums.mozillazine.org/viewtopic ... &t=3067141
I am rather surprised no-one else seems to have encountered this issue - all calendar events disappearing.
Today - I went back and located one of the emails with the .ics attachment that caused the issue... and it looks like just reading that email is enough to trigger the problem (my fix is undone).
It looks to me like I need to report it as a bug at bugzilla.