Light Fire is a firebending ability in the Avatar Mod that allows you to ignite the block you're currently looking at. It doesn't have a lot of uses in combat but it can come in handy if the player doesn't have a flint and steel.
As with all other abilities, there are two choices for Light Fire's level IV upgrade.
Flame Wall: Create a line of fire in the direction you're looking
This upgrade is pretty useful when fighting against a horde of enemies. It lights on fire a row of four (4) blocks in front of you. It becomes more offensive than it was before. It is recommended if you want to use this ability offensively.
Wide Range: Light nearby blocks on fire as well
This upgrade comes in handy if you use Light Fire for burning down areas. It allows you to cast the ability once and ignite more blocks than before when you had to cast the ability several times for that same effect.