Flash Fire
From Pixelmon Wiki
In Battle
Flash Fire is an Ability that makes the Pokémon immune to Fire-type moves. After being hit by a Fire-type move, the Pokémon's own Fire-type moves will increase in power by 1.5×. This boost cannot stack with itself, although the Pokémon will still be immune to Fire-type moves after the boost is activated.
Outside of Battle
If a Pokémon with Flash Fire is in the first place in the party (even if fainted), Fire-type Pokémon spawn more frequently.