To collect fireworks, you will need to ask friends to send them to you. This is a bit tricky, because all three fireworks are not immediately available. Right now, you can send and receive the Red Firework only. On the gift page, you can see that the White Firework will be giftable in 1 day, and the Blue Firework will not be giftable for 3 days. This stretches the game out a bit and makes it so you cannot possibly finish the game on the first day. Sparklers can only be received from wall posts, but anyone can go in and share sparklers.
To access the 4th of July game in Zoo World, go to your home zoo and click on the red, white, and blue boat that is parked near the entrance to your Zoo.