Flamethrower

The Flamethrower is the Beam Combo for the Plasma Beam. When this beam combo is unleashed, it emits a 3 meter range continuous beam of fiery plasma. Being hit with this beam combo causes enemies to ignite, taking away massive amounts of energy at a continuous rate. Note that being a continuous weapon, it will take 10 missiles to activate and use 5 missiles per second for as long as it is active. It can be obtained after destroying a Bendezium gate in the Phazon Mines to access Storage Depot A. Like standard Plasma Beam fire, it is capable of melting ice and setting enemies alight. It is very effective for destroying enemies in large amounts, as sweeping the flamethrower across their ranks will dispatch them quickly and easily.

Metroid Prime Data
"The Flamethrower is the Plasma Charge Combo. You can sweep its stream of flame across multiple targets. Samus's Notes: Flamethrower is a Sustained Fire Charge Combo. It takes 10 Missiles to activate, then 5 Missiles per second afterward. The Flamethrower is most effective against multiple targets in an area."

Trivia

 * Samus's forward-air attack in Super Smash Bros. was called the "Flame Thrower" in the official North American guide.
 * The Flamethrower cannot be used underwater. If it is being used as Samus jumps into water, it will immediately cancel.