Fighting fires was not just a job to Assistant Chief Donald J. Burns but his passion, one that rubbed off on many of his colleagues within the New York Fire Department.

Burns, who remains missing since the Sept. 11 attacks on the World Trade Center, was memorialized during a Mass yesterday in St. Patrick's Cathedral, where he was hailed as an "exceptional" leader within the department.

A 39-year veteran of the department, Burns, 61, rose from firefighter to citywide borough commander, a job that required him to be at all major incidents in the city during his 24- hour shifts.

During his career, Burns' ability to learn from his mistakes and use them to improve firefighting skills was apparent to most of his colleagues, who respected him as a brilliant tactician and strategist.

"Anything you ever wanted to know about the Fire Department, he would have the answer," said Deputy Asst. Chief Albert Turi after yesterday's Mass. "If you could be half as good a chief as he was you'd still be a good chief."

In talking about Burns at the service, Mayor Rudolph Giuliani and Fire Commissioner Thomas Von Essen used words like "hero" and "patriot" to describe Burns.

"His experience was unparalleled," Von Essen said.

In front of the cathedral, a large American flag was hung from a fire truck over the heads of several hundred uniformed firefighters lined up along Fifth Avenue.

Burns' family led the procession into and out of the cathedral, to the sound of solemn music played by the Fire Department's bagpipe band.

Burns, who lived in Nissequogue, is survived by his wife, Betty, and three children, Laurie, 37, Michael, 35, and Patrick, 29.

He began his career in 1962 in Brooklyn and rose through the ranks in various positions until 1997, when he became a citywide tour commander.

"When you were with him, you feel there's no fire you couldn't conquer," said 51st Battalion Chief Howard Carlson, based in Richmond Hill. "He was exceptional."