I observed a bright neon green fiery sphere shooting rapidly across the clear sky over Billings, MT.I am a former Police Officer and retired Fire Chief and currently work as an armed mobile security officer. On the above date and at the listed time, I was on patrol with my partner and just exiting the parking lot of a hospital that we patrol.My partner was looking down at the tablet in which she was entering patrol information and I was looking eastbound out the window of the patrol car. I observed a bright neon green fiery sphere shooting rapidly across the clear sky traveling from WNW to ESE.The sphere disappeared from view behind a nearby building. I heard no associated noise and saw no further activity. At first, I thought it must be some type of fireworks, but it was larger than any firework I’d ever seen and there was no launching sound, traveling sound or any other balls of light. Unfortunately, I had just minutes before shut off my dash cam which would have been sighted right towards the event.I let out an exclamation and my partner asked what was going on. I asked her if she’d seen what I’d just seen without being specific and she replied no. We cruised the area for a while looking for signs of fireworks or anything similar and there was nothing and no one in the immediate area.The total sighting was no more than 3 seconds in duration and I have never seen anything like it before. I have absolutely no idea as to what it was I saw.