I've upgraded to seamonkey 2.46 cand. build6  and fortunately lightning 5.1b1
was updated semi-automatically.
The basic calendaring functionality works. I can add events etc.
But when showing ICAL messages received in the mail Inbox there's no active
select button anymore (e.g. to accept an invitation). Only the small black
triangle for expanding the buttons is shown but the button list seems empty.