Specifically, does a Pokemon with Flash Fire take damage the first time it is hit with a Fire move AFTER which it is immune to Fire moves and gets a damage bonus? Or the first time it is hit with a Fire move will it negate that damage immediately?
I would prefer confirmation from someone who has observed this in game.
Flash Fire makes the Pokémon immune to Fire-type moves and will activate when hit by one. When activated, the power of the Pokémon's Fire-type moves is increased by 1.5×.
While subsequent hits by Fire-type moves will not provide further increase in power, the Pokémon remains immune to their effects.
This means that a Pokemon with Flash Fire is always immune to each Fire-type move, the only effect that needs you to be hit by a Fire-type move to be active is the +50% boost on Fire-type attacks.