You know what? music is very emotion-focused, more so the comtemporary ones. And dependending on the genre...
One well-behaved explanation would be she compares herself with the fireworks in the fourth of July, obviously a day of parties in the U.S.
Other possible explanation would be a double entendre in wich she compares herself with the fourth of July. But then, you know, in movies what is happening
when all of sudden there are fireworks shooting across the sky, and people would go ah ah ah. And she still sings that he (you, in the song) shoots across the sky (very double entendre, if any):
'Cause baby, you're a firework
Come on, show 'em what you're worth
Make 'em go "ah, ah, ah!"
As you shoot across the sky-y-y
Or none of above, maybe she is the one to explain it properly.
You will leave them in awe - Você os deixará impressionados, deslumbrados, admirados, espantantados...acontece com os dois tipos de "fireworks" citados...