Firefighters quickly went to defensive operations as flames tore through the 3½-story, wood-frame building on North 7th Street after igniting shortly after 2 a.m.
Most of the interior collapsed within an hour.
The fire captain was taken to St. Barnabas Medical Center in Livingston with burns that weren’t considered life-threatening, authorities said.
All of the residents got out OK and no other injuries were immediately reported in the three-alarm fire, they said.
City officials said that 49 residents were displaced and receiving temporary lodging, food and clothing from the American Red Cross.
The cause wasn’t immediately determined.