Earl Firefly(a.k.a. "The Professor") was the husband of Gloria "Mama" Firefly and fathered two children with her; Tiny Firefly, and (possibly) Rufus Firefly. According to Gloria, he was a very nice and kind hearted man, until he snapped one day. In a brute rage, he doused their son Tiny in gasoline and lit him on fire, resulting in his deformities.
House of 1000 Corpses
Until the climax of House of 1000 Corpses, "The Professor" is only mentioned by Gloria as the father of Tiny, and the reason for his burns. He resides down in the Catacombs with Dr. Satan and his patients. When Denise discovers this area, The Professor chases her throughout the Catacombs with a large axe. However, Denise used Earl's bad eyesight to her advantage, and he hits a support beam connected to a large fountain... The foundation falls, hits Earl's head, believing to have broken his neck. He is believed to be dead, however when Denise is captured once again, she awakens on the operating table to see Dr. Satan above her, and The Professor in the room, therefore meaning that he must still be alive, unless the possible theory that Denise had hallucinated the final events were true.
The Devil's Rejects
Though The Professor has no actual role in The Devil's Rejects, it is probable that he is still alive, in which he would play a part in the resolution to the story as being the final surviving member of the Firefly family.