Can I somehow block a spell that targets a zone and damages all creatures within it, like Firestorm?
Yes, Walls will block ranged spells. From the rules (page 17) regarding Spell Range:
Note: If there are any walls between your spell caster and the target, the walls may block your spell (see “Walls” on page 21).
You cannot cast a spell or make a ranged attack through a wall that blocks LoS. (The icon that indicates this is a picture of an eye with the red diagonal line through it)
An example is also listed on page 21 diagramming Line of Sight for a spell.
When you count range to your target, you must also check if you have clear line of sight (LoS). Draw an imaginary straight line from the center of your creature’s zone to the center of the target’s zone. LoS is blocked if the line crosses through a zone border that has a wall on it with the blocks LoS trait. Walls do not normally block LoS if the line crosses diagonally through the corner of a wall border, as long as at least one side of that corner does not block LoS.
Normally, there is the commonly used spell Block, that allows you to avoid a targeted attack. This doesn't work though, because Firestorm is a Zone Attack and as the Unavoidable attribute.
Block - When this creature is attacked, you must reveal Block during the Avoid Attack Step. Block counts as a Defense, and the attack is avoided. Then, destroy Block. If the Attack is Unavoidable, destroy Block without effect.
Zone Attack or Spell - This attack or spell affects all creatures and conjurations in the target zone, without actually targeting them individually. Does not affect walls on the zone’s border. Zone attacks have the Unavoidable trait and make a separate attack against each object in the zone. See “Zone Attacks” on page 27.
Page 27 of the rule book actually shows the Firestorm spell within its example sidebar, and indicates that:
... Zone Attacks always target a zone, never a creature or object. Therefore, abilities and traits that allow a target to react when it is the target of an attack do not apply against a Zone Attack. Zone attacks always have the Unavoidable trait, so Defenses have no effect against them. ...