Jeff Lundy copped the plea to the lesser charges during a pretrial hearing in Brooklyn Supreme Court.
Lundy sparked the May 22, 2001, fire on 17th Street in Windsor Terrace that killed Richard Lang, 63.
Lundy told cops he smoked crack and placed a smoldering crack pipe on a mattress in his first-floor apartment, touching off the blaze that quickly engulfed the dilapidated, three-story house.
The victim's son, Richard Lang Jr., 37, worked with more than 100 fellow firefighters to battle the two-alarm fire.
Lundy was so high on crack when the firefighters arrived that they had to wait hours before they could begin to question him.
Lundy had been charged with murder and arson in the fourth-degree.
As part of the plea deal, he will be sentenced to four to 12 years in prison.
