The "fireworks" mode will hold the shutter open longer than normal in order to get the light streaks

. When you do this in daylight and shoot at a moving object the light that was behind the object while the object moves along its path will overflow the light reflected from that moving object on the film, or in this case the area of memory with a digital camera.

It is actually a useful tool if you want to shoot a street scene and don't want the moving cars to be seen. I once shot a picture of the Sidney Lanier Bridge in Brunswick  that was shot in a long exposure (Fireworks Mode) and none of the cars moving on the bridge appeared in the print.

Try taking a picture of a airplane when the lights are on and the sky is dark,in the auto mode and then in the fireworks mode, you will see what I mean.