Throughout the seasons, the characters can be heard talking about a firefighter name Paddy Brown, and the different things he did. Paddy Brown was a real-life firefighter who gave his life on September 11, 2001.
Jack McGee (Chief Jerry) was a real-life FDNY firefighter in the Bronx, before moving to Hollywood to become an actor.
Throughout the show, Tommy is seen wearing a "Leary Firefighters Foundation" t-shirt. The Leary Firefighters Foundation is a charity which was founded by Denis Leary in response to the 1999 fire in Leary's hometown of Worcester, Massachusetts, which killed six firefighters, one of them being Leary's cousin. The LFF provides funding and equipment for fire departments in the United States.
Many of the extras were retired or active FDNY firemen who worked on the show on their days off. Most notable amongst these is Niels Jorgensen who eventually got considerable screen-time and small arcs as Niels, the chauffeur of 62 Truck.
In the season two episode, "&#!&", Sean Garrity remarks that Tommy (Denis Leary) resembles Conan O'Brien. Leary is O'Brien's cousin.