Ewelina Zimnicka and husband Jaroslaw were on their way to hospital when they got stuck in traffic in Ponders End in the early hours of Wednesday.
An ambulance was called but before paramedics arrived fireman Ross McLaren delivered the boy in the back of the couple's car.
Mrs Zimnicka said she was surprised her son Antoni arrived so quickly.
"When I went into labour at home I thought I had a lot of time as my first baby's labour had taken a long time. But then the contractions started coming quicker and I told my husband we had better go," she said.
"We set off in the car but we got stuck in traffic almost straight away because of a fire. Both ends of the road were sealed off and there were fire engines all around us. My husband rushed out of the car and told police we needed help.
"The fire brigade put a barrier around us and helped me and, just as the baby was born, the ambulance crew arrived in time to cut the cord."
She added: "I was so surprised how quickly the baby arrived. He was healthy and he cried almost straight away. The firemen were excellent, I can't thank them and the ambulance crew enough."
Mr McLaren said when he arrived to help, the infant's head was already visible.
He was helped by colleague Richard Hall.
Amanda Salmon, of London Ambulance Service, said: "Mum was holding the baby when I arrived. The cord hadn't been cut yet so I did that."
Mr McLaren, who has been a firefighter for 15 years, said he had no first aid training but his wife watched the TV programme One Born Every Minute.
"It has definitely been one of the highlights of my career and it's not something I ever expected that I would do in my life," he said.
"Someone said to me the other day that as firefighters we usually enter people's lives at their worst moments, so being there at such a joyful occasion makes a nice change."
A firefighter delivered a baby when a woman went into labour near the scene of a blaze in north London.