Fireball is a firebending ability that creates a fireball with great destructive capabilities in your hand. The fireball can be thrown (indicated on the crosshair) by pressing left click. When it hits a block or an entity it explodes, doing blast damage and setting things on fire.
As with all other abilities, there are two choices for Fire Ball's level IV upgrade.
Focused explosion: Can sometimes destroy obsidian
Focused explosion is an upgrade that suits better a firebender who likes to destroy things. It doesn't improve the offensive part of the ability at all but allows you to destroy blocks.
Charged explosion: Takes a few seconds to charge, larger explosion on impact
This one a great upgrade because of its high damage potential. You simply need to wait a few seconds and the ball will start to grow bigger and bigger, when thrown it will do massive damage to the area.