So I came across this (and a forge) dragon today in the shop after a friend pointed it out to me. It suggests in the description that this is a limited time only dragon so that hints that there are restrictions to it. When trying to breed other limited time dragons I am wondering what other requirements may be on this dragon to evaluate its worth in breeding. I can see that it is a Fire and an Air combination so I am looking at what the Other requirements are to it.
According to the Dragonvale Wikia:
The Firework Dragon can be bred by selecting a fire dragon to mate with an air dragon at the Breeding Cave/Epic Breeding Island. Breeding a fire dragon with an air dragon may also result in a Blazing Dragon.
It has a Breeding time of 6 hours (4 hours and 48 minutes on an upgraded island), and an Incubation time of 6 hours.
Note that you must be level 14 or higher in order to breed a Firework Dragon.
According to the in-game tool tip, the Firework Dragon will be available until July 20th. Apparently it can still be bred, though it doesn't appear in the shop anymore.
Having just managed to breed two of them in a row, I wouldn't worry too much about the limited availability.