It was an unusually slow day at the firehouse on Eighth Avenue and 48th Street, and the men put on a pot of coffee and got to talking about, of all things, mustaches.

While the hair above a man's upper lip might seem a topic of little consequence, it has always carried great significance among the men in the New York City Fire Department. Like red suspenders and Dalmatian dogs, mustaches are a tradition, a part of the lore and the code, a badge of the brotherhood.

No scientific study has ever been undertaken, but it is safe to say that a high percentage of the firemen have mustaches. ''We're guessing 40 percent of the guys have mustaches,'' said Assistant Fire Commissioner John Mulligan, who heads the department's public information office and who recently investigated the matter.

Mr. Mulligan, in keeping with tradition, has a mustache, which he grew after joining the department. So do the other three men in the public information office.