My brother in Christ you have a gun.
Impact grenades are best counter, once they start burning you your aim goes to shit.
Personally I enjoy killing enemy Flamethrowers with my own Flamethrower (plus 35% vitality and quick extinguish) 
It becomes a matter of who can melt the other person first (and the rest of their squad), and being able to self-extinguish in time to survive and carry-on.
The best is burning paratroopers before they hit the ground. It’s hilarious to watch the player as he’s angrily switching from one soldier to the next, trying to get an opportunity to kill the flametrooper. But it never works 
snipers and AP mines have entered the chat
HVAR and M8 have entered the chat.
Not useful on every point
Let the meatshields go in first then burn the enemy along with them.
HVAR and M8 vs litteraly anything:

tank 
Flamethrower kills tank too
Tank kills flamer first
And often sits in the grey zone
Well it´s usually hard to defend the cap fom the grey zone so that problem takes care of itself.
Difficult to shoot a flame trooper when they are burning you from the other side of a wall.
It would also help if you could have some soldiers fallow you around, so when you die to flamethrowers which are visible from outer space with how bright they are now, you can switch to them and throw granade or just shoot them from more than 30m.
Nah, better have every soldier not even react to 1.5s of jet of fire melting their face.
I want a volunteer to take flamethrower for just one second to face irl, they can even have wet clothes
before die burned i try to shoot or stab that flamethrower
Pretty damn satisfying catching the flamer on the end of your bayonet while you burn
…then quickly put out flame perk.