So the answer's clear and unambiguous, then? 
Actually, I like Dracosauve's first answer, thematically - the creature in a square of the wall of fire is literally surrounded by fire, and is therefore indistinct or at the very least the bright flames make it painful to look at - therefore has concealment.
Cheers, Al'Kelhar

Actually, I like Dracosauve's first answer, thematically - the creature in a square of the wall of fire is literally surrounded by fire, and is therefore indistinct or at the very least the bright flames make it painful to look at - therefore has concealment.
Cheers, Al'Kelhar