I came up with this idea, because my area is on a burn ban, so I couldn't do fireworks. So I went to play Minecraft, then I got the idea, I tried multiple cannons, and then came up with this. It's not to complicated. Just so you know, I DID use a mod. Too Many Items 1.7-1.7.2 by Marglyph.

Step 1: Supplies

The supplies you need are:
Minecraft
A flat area that is atleast 11x11
2 stacks of any non-flamable material.
39 redstone
4 buckets of water
1 bucket of lava
1 sign
2 pressure plates
and ALOT of TNT.

Step 2: The Base of the Contraption

This is the main body of the contraption.
Preview picture. Finished base
1. Make this, the long areas are 5 blocks, and the small is 3.
2. Add onto it. It is the same dimensions.
3. Repeat on the other sides, again, same dimensions.
4. Put 1 block into the middle of it.
5. Put 2 blocks on top of the middle block. These will later be removed.
6. Put a block on the side of the top block you just placed.
7. Do the same to the other side.
8. ^
9. ^
10-13. Now to the blocks you placed in step 5. Remove them.
14. Finished base.

Step 3: Firework Lighter

This is what ignites the TNT (AKA firework)
Preview picture. Finished firework lighter.
1. Put a block on any of the inside corners like so.
2. Put a block on the floating block opposite of the block you just placed.
3. Put three blocks onto the back of the block you just placed like so.
4. Put 4 blocks on each side of what you made in 2 and 3.
5. Put blocks on the outside of what you made in 4, but don't put one on top of the one you placed in step 2.
6. Place a sign on the side of the block you placed in 2 like so.
7. Where you going to be placing the lava.
8. The sign should stop the lava.
9. Finished product.

Step 4: The Redstone.

The redstone..
Preview picture. All redstone placed.
1. Place all this redstone.
2. Add the stuff in the yellow box.
3. Place all this other redstone.
4. Place this pressure plate.
5. Place this pressure plate.
6. Everything placed.

Step 5: Adding TNT

Adding the TNT
1-3. This is one of the parts of the base. Place the TNT where it is shown.
4-5. Other parts with TNT.
6. Place this one. This is the "firework"
Now, for the fun part. Firing.
Proceed to next step.

Step 6: Firing!

This is the fun part.
1. Step on this pressure plate.
2. Then rush to step on this one, and jump off, and look into the sky.
3. Firing during the day.
4. Firing during the night.
5. SHEEEEEEEEEEP!
Hope you like my firework contraption. Thanks for viewing my Instructable!