On default settings I don't think would be too much of a different in what height a firejet could reach traveling directly upwards and the general height limits of waterspout and airspout. I definitely agree with you on that point though, it needs to be balanced.
The thing is though, that those kind of factors, when implemented, are usually configurable, so server owners can make them however they like. So whether or not the default settings are fairly balanced or not it should still be possible to achieve it.
I'm thinking a good way to implement it would be to be low range/speed up and down, but quick moving sideways. So in the ways air/water spout move at pretty slow rates horizontally but up/down fast this would be the opposite.
It also reflects the nature of the element. Flying with fire is sticking a rocket ship up your a##. You don't have a gentle element like water and air to cradle you, it should feel like it.
Mechanics
basics - So it would hold you 1.5 blocks in the air like an AirScooter that moved at 2/3 FireJet speed horizontally (or for balance sake and to make FireJet still relevant, slightly faster than sprinting) but in whatever direction you moved rather than the direction you faced.
Passive - You would want to have it just automatically raise 1 block up every 2 seconds of use (something slow) when standing still as if the bender is only just overcoming gravity., but no raising when strafing around as all of the power is going into horizontal movement.
Control set - And rather than have the shift and space control set that air/water spout has. The shift key is reserved for canceling the ability. Then the space key can be used to raise faster than the normal rate when standing still, moving and rising is possible too. But this is at the expense of horizontal speed.
The position of the player would be based off the ground (like air/water spout). So if you try to dash over a ravine, you are gunna get #reKt.
The mechanic with pressing space could be used. The player loses horizontal movement speed, but gains the ability to move around without needing ground beneath. Which then be more like FireJet that does not care if you have ground beneath you or not. That would also make it feel like it deserves to share a slot with fireJet.
How it moves - Moving in a direction with this ability would give you very fast acceleration to top speed and quick direction changes but overall low top speed (its all about agility). The low top speed would give people a reason to still want to use fire jet.
Getting hit cancels - Unlike air/water spout, fire is a volatile element. So being hit should cancel the effect. Besides, if you cant do anything with that level of agility you don't deserve it anyway.
Just an idea, fire trails (see below). Whether thats the last 5 blocks you traversed or just entities you go past/over.
^Gosh damn Azula, calm down. This is during sozins comet but this chick does this crap all the time. This is the strafing kind i had in mind.
^Dashing around. Wrecking lives.
^Non sozins comet example, going out to beat up some punks in the earth kingdom. Note that she didn't really leave the ground. This is why i think the "space to go up, shift to cancel, but direction keys to accelerate/change-direction on a dime" is the way to go.
Usage examples
Its used for a lot of the same reasons as air/water spout but it should be fire style. It should be distinctive. That would be used as a fast paced strafing tool so you could face your target and pop off a few fire blasts in their face. Before the effect runs out and you get dropped on your a##.
This ability should be able to be used to move in a controlled manner. If you want. Whatever.
It could work for times when you want to move up slowly, with the ability slowly lifting you (faster if you press space) to reach that last 3 blocks up to the roof. (No firejet, i dont want to shoot 50 feet in the air and U-turn back to the earth)
Or across ravines (but not into the creeper 5 blocks to the other side), across holes underground (but avoid hitting the lava just next to it), or use it to land on 1x1 block pillars(without falling to your death).
^This is kinda how i envisage the space function working. Press space to not need ground underneath, sacrifices horizontal speed but at least you dont get reKt trying to dash over that ravine.
Or the horizontal sprint to escape a mine while not moving at the eye blazing speed of firejet.
Tell me if you agree with this post. I think it fits into the ethos of firebending better than anything i have read so far, but i cant read it all.