Minecraft wiki entry for Development_versions, associated text: While flying using elytra, using a firework will give the player a boost for a certain amount of time. This will emit a particle when in use.
Fireworks stay at the players location until they disappear or explode.
Longer flight duration on the rocket will make the boost last longer. The player can increase the flight duration of a firework by crafting it with gunpowder in the other slots.
Fireworks with an explosion will deal damage at the end of their flight.
The right-click on a block action for the fireworks is prioritized over the right-click ability in midair. This simply means that when the player looks at a block while flying and right-clicking, the rocket will shoot regularly, instead of being used as a boost.
Firework explosions now hurt surrounding mobs and players.
The damage radius of the explosion is about 2-3 blocks and is the same for any shape (large ball, creeper, burst, etc.) and the damage decreases with the distance to the explosion.
The damage amount at close range is about 5 without armor on, and with full diamond armor, it equates to about 1.
Death message is "[player] went off with a bang".