Oracle Text

As an additional cost to cast this spell, discard X cards.

Firestorm deals X damage to each of X targets.

Card Rulings

4/1/2008 For example, if you discard four cards, you must target four different creatures and/or players. Firestorm will deal 4 damage to each of them.
4/1/2008 The value of X can’t exceed the number of cards in your hand or the number of available legal targets, whichever is smaller. X can be 0.
4/1/2008 The same permanent or player can’t be targeted more than once.