The FHQ is fine IMO. You can't repair the building so it will go down eventually (even if you only use regular mortars).
The Mortar HT counters it perfectly, it arrives 2CP later but that shouldn't be a huge problem.
Early game, this doctrine is a pain in the a** to play against but once you have the unit you need to counter it...well, your opponent does not have any good abilities to use (except for the incendiary strike).
flame barrage is 30muni
cost of grenade > 80 fuel FHQ
FHQ costs 60 fuel, flame barrage costs 45 muni.