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 50%. While subsequent hits by Fire-type moves will not provide further increases in power, the Pokémon remains immune to the moves.
Flash Fire will not activate if the Pokémon is protected from the Fire-type move.
Flash Fire will not work while the user is frozen.
Instead of boosting Fire-type moves' power, Flash Fire now technically boosts the Pokémon's Attack or Special Attack by 50% during damage calculation if a Fire-type move is being used, resulting in effectively the same effect.
Generation V onwards
Flash Fire is now functional even if the user is frozen.