At 1800 Pacific time, a Cessna 172M veered off the runway during the landing rollout on runway 7 at Rosamond Airport and was damaged when the left gear collapsed. The pilot and two passengers were not injured. The pilot said the landing was normal and that the left gear collapsed when he braked to turn off the runway. An inspector detected tire signatures on the runway that were not skid marks and suggested the airspeed may have been too high to make the turnoff the pilot was trying to make. While reviewing the airplanes logbooks, the inspector noted a reference from a year ago that the firewall was wrinkled, complete with photographs. There were no logbook entries detailing repair of the firewall.