A Redditor has thrown up an image of the First Order Jet Trooper from D23, and he looks similar to the Flame Trooper if it had sex with the Snow Trooper.
The armor is definitely a blend of the those two First Order Trooper types, with the Flame Trooper design coming through the most. You can’t really see the jet pack in the shot, but he can definitely fly.
The markings on the chest are interesting too, because most of the First Order Trooper variations, like the Imperials before them, don’t have markings or designations on their armor. These guys seem to have some sort of special symbols that must signify that they can fly and are much more radical than their ground based compatriots.
This design was done my Michael Kaplan, and you can briefly see the Jet Troopers in action in the teaser for The Rise of Skywalker. If you blink you will miss them though, so looking at the static image above is your best bet for now.