Do cards like Firestorm or Tibbers affect facedown hidden units at a battlefield?

Ruling: Facedown cards at a battlefield do not take damage from effects like Firestorm or Tibbers. A facedown card is not actually on the board and is not even a unit until it is played to that battlefield.

Nuances:

• Facedown cards are technically in the "facedown zone" of that battlefield, not at the battlefield itself
• If a unit is face-up at the battlefield (already played), it will take damage from these effects
• Facedown cards can be played from the facedown zone for 0 energy as a reaction, and units must be played to the corresponding battlefield