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
Linked issues
duplicates
Comments

What`s the issue here ?
as soon as he answers my question we will find out.

Let me guess, this is one of the "I-take-vanilla-Minecraft-and-mess-around-with-third-party-tools-now-my-minecraft-is-broken"-issues ?
probaply

Nevertheless, other mods gave the hint to the better description in MC-4236
when you hover your mouse over the firework, what does it say besides: "firework rocket"?