In the following city (North Qeynos, Kelethin, West Freeport, Neriak, New Halas, Gorowyn) you'll find an npc called "a fireworks coordinator".

Before being allowed to speak to him, he will ask you to find a "magical flare". Simply claim your firework in SOE station cash under the "summer fireworks festival" section. Then speak to him to get a quest that requires you to launch 10 fireworks. (Click on the fireworks launchers located throughout the city to get an update.)

As a reward, you will get a fireworks launcher for your house. Either quest is repeatable; you don't get a different launcher when repeating the quest in the same city, but you will receive a different launcher from coordinators in other cities.