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.

3 Answers

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.

2

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.