How do I Program Fire Breath?

If you want to simulate the Fire Breath quirk instead of using roblox's fire particle you could use custom particles. Just remember, you won't be able to make the particles deal damage, I suggest making a hit box the same size as the particles and them make that hit box deal the damage.

I made a custom particle that I think resembles Fire Breath: (Note: it does not move with the player or face the same direction as the player, it just creates the particle at the player head when E is pressed)

local plr = game.Players.LocalPlayer
local char = plr.Character
local hum = char.Humanoid

function onKeyPress(inputObject, gameProcessedEvent)
	if inputObject.KeyCode == Enum.KeyCode.E then

                -Create Part
		local fire = Instance.new("Part",workspace)
		fire.CFrame = char.Head.CFrame
		fire.Orientation = Vector3.new(0,0,90)
		fire.Size = Vector3.new(0.5, 0.5, 0.5)
		fire.Transparency = 1
		fire.CanCollide = false
		fire.Position = char.Head.Position  + Vector3.new(0,-1.5,0)
		fire.Anchored = true
		fire.Parent = char.Head
		
		local part = Instance.new("ParticleEmitter", fire)
		part.Color = ColorSequence.new(Color3.new(225,0,0),  Color3.new(253,95,15),Color3.new(17,17,17))

		part.LightEmission = 0.3
		part.LightInfluence = 1
		part.Size = NumberSequence.new(1.09,1.06)
		part.Texture = "rbxassetid://566194599"
		part.Transparency = NumberSequence.new(0,0.4)
		part.Name = "PartleEmitter"
		part.LockedToPart = true
		part.Lifetime = NumberRange.new(3,4)
		part.Rate = 100
		part.Rotation = NumberRange.new(50,50)
		part.RotSpeed = NumberRange.new(50,50)
		part.Speed = NumberRange.new(20,20)
		part.VelocitySpread = 20
		part.Parent = fire
	end
end
 
game:GetService("UserInputService").InputBegan:connect(onKeyPress)