The firefighter was struck by the air conditioner that apparently fell from the window of the 85th Street Bensonhurst home sometime after 9 a.m.
FDNY members rushed to his aide after the firefighter was knocked unconscious.
He was taken to Lutheran Hospital, where he was stabilized.
He is expected to survive.
The firefighter's helmet was cracked and sliced open by the AC unit.
Five other firefighters suffered minor injuries from fighting the blaze.
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