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.
From Bulbapedia:
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.
In all generations, if a pokémon has Flash Fire, it is effectively immune to all Fire-type moves, both damaging moves and status moves (Will-O-Wisp).
In generation 3 and 4, however, if the pokémon with Flash Fire was frozen, it would not activate and the pokémon would take damage. This was changed in gen 5.
Flash Fire makes you immune to any Fire move. It also boosts the power of your own Fire moves by 50% after being hit by one, for as long as you stay in.
