A firefighter was injured and a house left uninhabitable Sunday after a Saturday night fire burned an unused mattress in an attic of a house near downtown Elgin.Batallion Chief Terence Bruce said in a release a relative of a resident at the home on the 400 block of South Street reported the fire at 7:23 p.m.The fire was brought under control quickly, but a firefighter suffered an electric shock when he came into contact with an exposed wire.Firefighters cut a hole in the roof to attack the fire, which had gotten between roofing materials.