I too thought it was a small fire, but from a higher vantage point, the angle I had made it look more like one of the firework pots had kept on burning. But they are on fireproof mats so no panic, I noticed that the attendants were in no great rush to extinguish it, but they did after a couple of minutes.
Having said that, I couldn't enjoy most of the display, for fear of watching my kiosk (and others) getting showered in fiery sparks all the time! Nerve wracking stuff.