What I expected to happen was I would have a firework spawner.
Describe what you thought should happen here

What actually happened was it spawned a firework but then it disappeared after it spawned
Describe what happened here

Steps to Reproduce:
1. Make a firework
2. Place the firework so it starts to fly up
3. Quickly exit out of Minecraft
4. (If you need help with this ask SethBling) Open up McEdit and select a box around the Firework entity
5. Then go to filters and choose create spawner the click filter
6. Go back into Minecraft and check the spawner