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.
ALSO SEE: A firefighter sustained a minor injury falling down stairs during a destructive blaze that blew through the rear of a multi-family, mixed-use Garfield building late Friday into Saturday.
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