If you put on an elytra and fly, use can use the firework to get to a certain fixed speed. If by gliding down you get a speed higher than what a rocket can give you, and you use a firework, you will lose speed. This is because the fireworks just set your speed without checking if you're above it, and slowing you down

Reference video: https://youtu.be/HWKXpOIhgTc