The cars collided on Paterson Plank Road at Congress Street in Jersey City just after 2 a.m. Thursday.
Three others are being treated at Jersey City Medical Center.
The off-duty firefighter was not driving, and other people in his vehicle were injured.
The firefighter has been identified as Matt Nierstedt. The JCFD posted a photo tribute of him on their Facebook page:
Nierstedt had been with the Jersey City Fire Department for one year. He was one of 19 military veterans sworn into the fire department at the same time in a unique ceremony.
His father was a Jersey City Police Department captain and his uncle was a Jersey City Fire Department captain.