I think the song’s title, firework, is like us who are worthy but we didn't realize and take an action to do something for our life, If we do not begin to make something, it is like fireworks which do not ignite.
So, firework means our life that should begin to do something and show everybody that how we are worthy. The speaker of the song encourages everybody who had a hard time and gave up trying to do their best, to be confident, to be themselves, to be brave to face hard things, and live fullest life.
We can see from the6thstanza of this song, and it is repeated again in the 12thand the 14thstanzas, which is the important meaning of the song