I think dispensers are good enough. It would also be hard to code (if you had fireworks and arrows in your inventory, how would the game know which to fire?). A firework gun would be better, and even that wouldn't be great.
I don't particularly like the idea but in defense of the code it may be doable. You can set a priority so that it searches for "ammo" type items on the hotbar first in order from left to right before going into inventory starting from top left to bottom right. Kind of tedious at this level but it should work.
If this would be added, I have a feeling it would amount to adding the explosion and trail effects to a regular arrow.
Not particularly fond of this one since the bow won't ignite the firework anyway, and as others have said, dispensers work fine.
...and now that I mentioned ignition I suddenly can imagine placing individual fireworks on the ground and lighting them with Flint n Steel to set them off.
OMG! I should be able to launch these with a Bow!
So, I figured you could use them to fight mobs in addition to launching them up in the air at will.
No support.
MAGIC SPARKLY MISSILES
LOL just kidding
Though missiles would be nice though
http://www.minecraftforum.net/topic/1698011-updated-pix-gryphons-balanced-flight-for-survival-mode/
Not particularly fond of this one since the bow won't ignite the firework anyway, and as others have said, dispensers work fine.
...and now that I mentioned ignition I suddenly can imagine placing individual fireworks on the ground and lighting them with Flint n Steel to set them off.
It always takes from the smallest/closet right justified stack.