You can find the culprit along the Road to War. The monster in question is a Skata’ne. Defeat it to receive 5 fireworks. You can return to the Fireworks Boss or back to Funicula. If you hand them back to Funicula you gain 7 fireworks so it is better to be honest. After giving them back, go and visit him in his new shop to regain the fireworks and your reward.