Originally Posted by Rockstar Support
Question: I bought the Fireworks Launcher during the GTA Online Independence Day Special. Will I be able to still use it after the end of this holiday period?
Answer: The Fireworks Launcher is a special holiday item and does not function like a traditional weapon. Once purchased, the launcher will be owned permanently and will remain in your inventory during non-holiday periods as long as you have special fireworks ammo in possession. Outside of designated holiday periods, the weapon will be hidden in your inventory once your fireworks ammo has been depleted.
The Fireworks Launcher along with its ammo will be available again during holidays and other special events when fireworks are traditionally let off. At the beginning of any holiday or special event where fireworks are available, players who purchased the weapon previously will receive one free full refill of ammo per qualifying event in order to make sure players can celebrate.
Of course, what those holidays and special events are, who knows. Fireworks can be set off for anything anymore!