The fire began about 2 a.m. in the kitchen of a home on Welldow Lane, where it spread to upper floors and two adjacent units, a battalion chief with the Fullerton Fire Department said.
The occupants of the apartment - four children, two parents and a grandmother - were asleep at the time. The family later said the flames were ignited by a faulty microwave.
News video showed dramatic flames emanating from a window of the second-floor dwelling.
One of the responding firefighters fell about 10-20 feet from a ladder, according to the battalion chief. The fall left his face visibly bloody before he was placed on a stretcher and carried to an ambulance by his colleagues.
He was transported to UCI Medical Center, the fire department said, and is expected to be released sometime Sunday.
The blaze was extinguished within 30 minutes. The occupants of the home escaped without injury.