Can Fox Fire target units on other battlefields when played from hidden?

Ruling: No, when a card is played from the facedown/hidden zone, it adds the text "here" to the card effect, restricting it to the current battlefield.

Nuances:

• This "here" restriction applies to targeting requirements
• Cards that don't target (like Zhonya's) follow different rules
• Some cards like Stand United may have both targeted effects (restricted to current battlefield) and area effects (can apply everywhere)
• Gear cards cannot be played to battlefields even when played from hidden