They've been in our Airbus narrow bodies since late 80s.
Older heaters were electric heated tubes with air being drawn through them, then exhausted around the door "threshold" area under and around the floor.
I think the newer ones are actual electric elements in pads on the floor but I'd have to check the manuals.
They actually make a big difference keeping ice and nastyness from accumulating on the seals, and really keep check on any cold draught from the cabin door.
Switches are on the 2000VU panel or on the fancy touchscreen flight attendant panel labeled 'FPH ON/OFF' or something like that.