Pokémon FireRed and LeafGreen/Mt. Ember

In this area you can catch the legendary bird at the top of the mountain, and near the bottom you can get the Ruby Gem which is necessary to complete the network machine and be able to trade with Ruby/Sapphire.

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 HM 06 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, , , in the grass, and  while surfing. Continue to climb up the mountain and enter the cave until you reach the top.

At the top of the mountain you'll have to solve an easy puzzle 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!