Do flight regulations prohibit sleeping on the floor of an airliner?
That depends on the air carrier and the regulatory body they're certified by. Finland is part of the EU so I'm assuming that Finnair's certificate is under EASA aviation safety regs. Specifically, a quick Google search finds:

CAT.OP.MPA.165 Passenger seating:
The operator shall establish procedures to ensure that passengers are seated where, in the event that an emergency evacuation is required, they are able to assist and not hinder evacuation of the aircraft.

Finnair would also write their own SOPs to ensure they meet all the EASA regs.