The flames broke out after 10:00 p.m. Wednesday in the second floor kitchen of a 3-story apartment building on the 200 block of Linden Avenue.
Two firefighters were overcome by heat exhaustion battling the blaze.
They were treated at Christiana Hospital and released.
The Delaware State Fire Marshal's office determined Thursday that the fire was caused by unattended cooking and ruled it accidental.
The blaze caused an estimated $10,000 in damage to the apartment.
A neighboring home sustained smoke damage.