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Volcarona-Good setup sweeper with Quiver Dance and great stats. Definitely use this.
Charizard-Immune to Ground moves and has Focus Blast to (70% of the time) hit Rock types, but compounds a 4x Rock weakness with Volcarona.
Chandelure-Highest Special Attack of Fire types in Pixelmon, also has Energy Ball to nail Water types. It's slightly slower than it would like though.
Blaziken-Strongest physical Fire type, but Hi Jump Kick is difficult to obtain legitimately and can be taken advantage of by Ghost Pokémon or Protect. Get Speed Boost if you can, but if not then you're better off using Infernape.
Camerupt-STAB Earthquake and immune to Electric moves including Thunder Wave. Very slow with mediocre defenses and only slightly above average offenses and has a bad typing leaving it 4x weak to Water.
Magmortar-Access to Thunderbolt and Focus Blast to cover weaknesses, nice Special Attack stat. Average Speed, slightly faster than Chandelure.
As for my own suggestions of additional Pokémon to consider:
Ninetales-If you can find a Drought Ninetales, automatic Sunny Day setup is amazing for your team, halving Water move damage, powering up your Fire moves, and negating SolarBeam's charge turn If you can't find one with Drought, don't bother using Ninetales.
Moltres-If you're allowed to use this, it's stronger than Charizard with the same typing and a stronger Flying STAB in Hurricane, although it lacks Focus Blast and is slightly slower.
Infernape-STAB priority Mach Punch, more reliable STAB Fighting move in Close Combat, faster than Blaziken but has less power.
For a full list of Fire Pokémon in Pixelmon, look here.