Based on my argument and the definition of a weapon, fireworks as they are, are not weapons, as their purpose isn't that of a weapon's. However...
You kind of said it here, a more accurate statement would be "something can be used as a weapon" as opposed to "a weapon is only a weapon if you make it one." This applies to both your baseball bat as well as fireworks mentioned above, and anything else for that matter.
On the same note, a weapon can be used as a non-weapon, such as using firearms for sport. However, because of firearms' main intended use, they are to be considered weapons, in my opinion.