[ibb.co]

You have a couple of choices to unlock all Fireworks

Hard Mode:
if you wanna find it yourself
Gameversion: 2021.6.1
Town: 3 Smokeballs(purple, yellow and green), 1 Spotlight and Tim
Ranch: 1Rocket(Foxy), 1 Firecracker(Snapper(blue)), Tim and Karls Son
Flat: None
City: None

Easy Mode:
Here is a Youtube Video
and here you find Karls Son: Here

Easiest Mode:
Go to
C:\Users\%USERPROFILE%\AppData\LocalLow\Laumania ApS\Fireworks Mania
[ibb.co]
and exchange the file "UnlockedItems.json" with this one:
Download "UnlockedItems.json" from Google Drive[drive.google.com]

If you wanna lock them again just delete the file "UnlockedItems.json" and the game will create a clean new file.