The devastation of the 2001 attacks on the Fire Department is well known, from its 343 members killed that day to the psychological trauma and medical ailments sustained during recovery efforts at ground zero.

But harrowing conditions for firefighters, of course, did not end with the attacks. In 2007, Mr. Byrne fought the seven-alarm fire at the Deutsche Bank building, which killed two firefighters and injured more than 100 others.

Then there were the two children who “died in his arms” after being run over in Chinatown, said his father, Ed Byrne, a Long Island lawyer who has written a memoir about his son, titled “In Whom I Am Well Pleased.”

These and other experiences on the job left Mr. Byrne with post-traumatic stress disorder and severe depression.

“The term firefighter is a misnomer,” the father said. “He was fine with fire, but it was the constant death that got to him.”

Also, working odd shifts and night tours, “he would come home some mornings all wired and start drinking beer,” Mr. Byrne recalled.

After taking painkillers for a knee injury, the younger Mr. Byrne developed an opioid addiction that included stints at rehab clinics and psychiatric wards.