The balkan tradition of firing weapons in the air whenever celebrating something is alive and well in Serbia. Up to the late eighties, it was limited to weddings, when a special prize (roast pig's head or some such) would go to anyone who could shoot an apple hung on a pole on the highest tree in the yard.
But there came the nineties and Sloba's wars, and the weapons have spread around, and there's a certain percentage of people who think that there can't be a decent party (wedding, birth, serbian new year, and while there was a conscription, farewell to the rookie) without shooting a few dozen bullets in the air.
The culture being watered down, and the religious customs being largely forgotten, some of the customs are reinvented by the first and craziest bidder, so the shootouts have spread to just about any holiday or party, regardless of its inner meaning. And it's not just a serbian thing - nowadays, you can hear the shots even on the catholic christmas eve (no matter that it's also the protestants' and others - the dividing line between the catholic and orthodox world is right behind the corner, and any third denominations are marginal in the language).
Bullets and alcohol don't mix well, and every new year's night there's at least one accident - one bullet gets lucky when falling and hits something it shouldn't. Given the odds, it's a repeated miracle that only so few get wounded, and that deaths don't happen every year.
Most of the shots you hear aren't weapons, though. Firecrackers are more frequent - they are sold for a few weeks in advance, from little stalls just around any place with a sufficient number of pedestrians. They are bought mostly by younger teenagers, who then fire them on the sidewalk and pride themselves on the number of passers by whom they startle. It's not limited to that one night, though - they may start practicing few days in advance, and continue for a day or two afterwards, to get rid of the leftovers.