To see or use a flamethrower in the dream refers to destructive words that you or someone yields. These destructive words are the fire that have the power to deal permanent damage. For example, if someone is using a flamethrower to burn you specifically, the dreams refers to some sort of plot against you.
If the dream features a flamethrower is being used to burn off grass, that relates to the flame-thrower is burning his bridges and connections with people surrounding him. The destructive words are stupid and ignorant that will cause long lasting damage.