when you right click with fireworks on a block while flying with the elytra, it will place the firework too as well as provide a speed boost.
not sure if intended but seems like a bug to me.
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Can you provide a video?
Here is a video: https://youtu.be/0Tpta3dRZv4
May be related to MC-111480 again
Placing the firework on a block while flying will spawn the firework. Cannot reproduce that it will also give you a speed boost.