WoWDB

In Warlords of Draenor, this item will be added to the new Toy Box feature and no longer take up space in a character's inventory.  You will be able to "use" the item straight from the Toy Box interface.  The Toy Box will be account-wide and therefore so will this item.

It seems that of the most recent beta build, learning and adding Everlasting Alliance Firework to the Toy Box will also add  Everlasting Horde Firework to the Toy Box.

I believe this is intended - and not a bug - because of two reasons:  Faction-changing the character that has the firework in their inventory will also cause the firework to change to its opposite faction, too.  Also, the Toy Box is account-wide, so it'd be a bit silly not to recognize that many players' World of Warcraft accounts have characters of both factions.

EDIT: The toy is now available for everyone!  Players can now buy it for 25 x  Coin of Ancestry while  Lunar Festival is active.