Does Firestorm deal damage to all enemy units at all battlefields, or do you choose a specific battlefield where all enemies take 3 damage?

Ruling: You choose one specific battlefield, and all enemy units at that battlefield take 3 damage.

Nuances:

• The phrase "at a battlefield" indicates you must choose a target battlefield, not that it affects all battlefields
• Cards that affect all battlefields use different wording (see Unchecked Power and Flurry of Blades for comparison)