Flamethrowers are weapons designed to project a long controllable stream of fire. In the First World War, they were used by footsoldiers, but since the infantry were very vulnerable, the concept was reworked with the result being new vehicle-mounted flamethrowers.
Flamethrowers are potent weapons with great psychological impact upon unprepared footsoldiers, inflicting a particularly horrific death of being burnt alive, and are also used against battlefield fortifications, bunkers, and other infantry protected emplacements. They are not so effective against armoured vehicles like tanks and are, naturally, useless against aircraft.
The most famous use of flame weaponry is the Chinese Dragon tank, which releases burning napalm that is extremely deadly to infantry, light vehicles and buildings. The Chinese also outfitted their basic artillery, the Inferno cannon, and their MiGs with napalm-tipped shells and missiles respectively. If enough napalm firepower is concentrated on the same area the result can be a deadly self-sustaining Firestorm ignitions able to incinerate everything in the vicinity. The Helix can be upgraded with the Napalm Bomb, which, when dropped, immediately create an Firestorm.
It should be noted that China invented an even more deadly substance named Black Napalm, with a classified mixture. Black Napalm produced an even more intense heat, with the result of far deadlier firestorms.