First tell the creature entry to get a material template (generally CREATURE_EXTRACT_TEMPLATE is used, but we can probably use FLAME_TEMPLATE in this case - it doesn't matter a great deal anyway, just affects the number of things you'll have to change):
[USE_MATERIAL_TEMPLATE:FIREBREATH:FLAME_TEMPLATE]
Now edit its properties to however you want them to be:
[USE_MATERIAL_TEMPLATE:FIREBREATH:FLAME_TEMPLATE]
[STATE_COLOR:ALL:RED]
[STATE_NAME:LIQUID:liquid flame]
[STATE_ADJ:LIQUID:liquid flame]
[STATE_NAME:GAS:flames]
[STATE_ADJ:GAS:flames]
#FLAME_TEMPLATE's native temp is 14000
[IGNITE_POINT:13500] #Just to see what happens. It should work, but if the game bugs out, you can just remove this line
[MELTING_POINT:13499]
#The other relevant stuff is already set by FLAME_TEMPLATE
[PREFIX:NONE]
Then the breath attack itself:
[USE_MATERIAL_TEMPLATE:FIREBREATH:FLAME_TEMPLATE]
[STATE_COLOR:ALL:RED]
[STATE_NAME:LIQUID:liquid flame]
[STATE_ADJ:LIQUID:liquid flame]
[STATE_NAME:GAS:flames]
[STATE_ADJ:GAS:flames]
#FLAME_TEMPLATE's native temp is 14000
[IGNITE_POINT:13500] #Just to see what happens. It should work, but if the game bugs out, you can just remove this line
[MELTING_POINT:13499] #Due to the LIQUID_GLOB bit in the breath attack, this can be left alone entirely, but I'll set it anyway
#The other relevant stuff is already set by FLAME_TEMPLATE
[PREFIX:NONE]
[MATERIAL_BREATH_ATTACK:LOCAL_CREATURE_MAT:FIREBREATH:LIQUID_GLOB]