A firework is a device containing gunpowder and other combustible chemicals which causes spectacular effects and explosions when ignited. A firecracker is a loud, explosive firework.
In other words, Firework is bunga api while Firecracker is mercun.
Playing with the safe version of firecracker aka bunga api gives me a warm and satisfying feeling inside. But I think I never play with mercun in my entire life.
Oh! Except the tiny, onion-like firecracker that sounds Pop Pop when it hits the floor. It comes in a small yellow box that has Pop Pop written on it. How I love that one.