Shortly after 4 a.m. Friday, JFRD and the Jacksonville Sheriff's Office responded to the fire at 2200 Jadestone Drive.
Firefighters found a man unconscious in the living room. The victim was pulled outside but could not be revived.
The victim's name wasn't released at the scene, but a relative who showed up about dawn was so upset and lashing out that she was arrested for breaching the peace and resisting arrest without violence.
The fire was quickly extinguished, but about 50 percent of the building was damaged, JFRD spokesman Tom Francis said. The adjoining duplex, which was unoccupied, suffered smoke damage.
"As of right now, we don't have any reason to feel that this fire is suspicious," said Detective Andy Redding, of the State Fire Marshal's office.