Pokémon FireRed and LeafGreen/Mt. Ember

In this area you can catch the legendary bird at the top of the mountain. To get to Mt. Ember, you have to use Surf. It's just north of One Island, but you'll have to go past Kindle Road.

Kindle Road contains a number trainers and the Ember Spa where you will find HM06 - Rock Smash. If you enter the middle pool within the Spa, it will heal all your Pokemon. Along Kindle Road you'll be able to find, , , and in the grass, and  while surfing.

Near the bottom of the mountain you can get the Ruby Gem which is necessary to complete the network machine and be able to trade with Ruby/Sapphire, but for now, the area is blocked by those two Team Rocket members. You'll have to come back later. For now, continue to climb up the mountain and enter the cave.

The cave is not very long, and once you exit you'll find yourself at the top of the mountain. You'll have to solve an easy puzzle with Strength to reach Moltres. Moltres will come equipped with Flamethrower, Endure, Agility and Fire Spin, making Moltres a very difficult fight. Make sure to bring a good amount of Ultra Balls!

Once you are finished here, head to One Island and talk to Bill. It's time to return to Kanto, and tackle the final challenge: The Elite Four!