"Describing what happened in the moments before the fire erupted, Dean said Helen G. was asleep on a couch around 10 a.m. at her home at 344 N.W. 41st St., when she smelled smoke. She opened the closet door and saw the smoldering foam mattress, which had come into contact with a light bulb."

Though they don't say explicitly, I'd guess it was a bare light bulb in one of those porcelain pull chain switch lamp holders. That someone forgot to turn off. This fire cost 5 lives.

Seattle Times story here