Historically speaking, it is very dangerous to say that something has never happened, but I can find no references which state that such use of "snipers" firing from balloons was a deliberate part of military tactics on the part of any country. I have recently reviewed numerous books and publications on the history of military use of balloons, and have found no instances of such use reported. I have little doubt that it was possible that a balloonist could have carried aloft a firearm and popped a few shots toward the ground, but the practicalities of balloon warfare in the early years mandated that this was never a planned use for balloons. The early use of balloons by various military forces (starting with the Battle of Maubeuge in 1794) was primarily for observation of enemy troops and movements. A few balloons were used in the 18th century to drop propaganda leaflets and explosives, without much effect. Balloons were inherently unstable platforms, bobbing in the wind at the end of long cables-- Shooting accurately from such a platform would have been totally impractical, not to mention that it would have been an extremely difficult and expensive way to get one or two riflemen into position to attack enemy troops at long range. Further, for their own protection, balloons were kept high enough to keep them out of enemy rifle and cannon range (and usually well behind the lines of their own troops), which would mean that any targets on the ground would have been at or beyond the furthest extent of practical range for firearms on board the balloon. Maximum range and unstable shooting platforms would make "sniper platform" use highly impractical. It is true that some WW1 balloon observers carried firearms, and were reported to have fired them at attacking aircraft, but again I can find no reference which states they fired at troops on the ground. An additional point to make in answering your question is that military balloons prior to World War 2 were generally filled with gas (usually hydrogen or lighting gas), and were thus not "hot air balloons". A popular but pretty comprehensive book which accurately describes the early use of balloons in war is "The Aeronauts", by Donal Dale Jackson.