Firestorm — ruling 1

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.

Firestorm — ruling 2

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.

Firestorm — ruling 3

The same permanent or player can’t be targeted more than once.

Firestorm : règles, fonctionnement et interactions (MTG)

Firestorm is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

As an additional cost to cast this spell, discard X cards. Firestorm deals X damage to each of X targets.

How it works : Firestorm

Original English text — Wizards of the Coast / Scryfall

#1April 1, 2008· Wizards of the Coast
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.
No other card is named in this ruling.
#2April 1, 2008· Wizards of the Coast
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.
No other card is named in this ruling.
#3April 1, 2008· Wizards of the Coast
The same permanent or player can’t be targeted more than once.
No other card is named in this ruling.

Interactions with other cards

These cards' text mentions Firestorm.

Temporal Firestorm

Kicker {1}{W} and/or {1}{U} (You may pay an additional {1}{W} and/or {1}{U} as you cast this spell.) Choose up to X creatures and/or planeswalkers you control, where X is the number of times this spell was kicked. Those permanents phase out. Temporal Firestorm deals 5 damage to each creature and each planeswalker.