The blaze broke out inside a six-story apartment building on Sedgwick Avenue just after 4 a.m. Thursday.
Firefighters found one man critically burned when they arrived.
Rushing up the stairs, officials say one firefighter fell through a stairwell from the third to second floors.
The 40-year-old firefighter was rushed to St. Barnabas Hospital to be treated.
He is listed as serious condition, as doctors conduct further tests. His injuries are not believed to be life threatening.
The male resident who was injured is in critical condition. He was burned to his hands and face and also suffered smoke inhalation.
Other residents were being treated at the scene.
Firefighters were forced to make numerous rescues, helping women and young children on fire escapes.
The residents will not be able to return home because officials say the stairwell in the apartment building is compromised.
Investigators are trying to determine what caused the fire.