The description for Inflammatory Actions says, “Kill a headcrab with fire.” I did this multiple times in the room with fire in Residue Processing but it didn’t count towards the achievement. I later looked at the community guide for getting all the achievements and it said you have to light them on fire with flares. I suggest either changing the description to read “Kill a headcrab with a flare” or fix it so killing a headcrab with environmental fire does unlock the achievement.
Only thing I can think of is the fire damage has to actually be attributed to the player. If it’s from the environment you can’t be sure if the player tried to do it, or if it just stumbled into the fire itself. In which case, then it’d be possibly having a random achievement pop up while you’re walking through a map rather than not getting the achievement for technically doing as it says.