Even though Pokemon FireRed takes place in the Kanto Region, the Pokemon from the first three Pokemon Generations are coded within the game since it was released during the 3rd Generation. This means that even though most cannot be found naturally in the Wild within Pokemon FireRed, all the Legendary Pokemon from the 2nd Generation and 3rd Generation can be traded for and obtained. Believe it or not, one piece of the Johto Legendary Beast Trio (Raikou, Entei, Suicune) can actually be found without having been a Pokemon Event or by using Cheats Codes.
After completing the Main Story and having defeated Team Rocket in the Sevii Islands, the Legendary Pokemon Entei will begin roaming the Kanto Region if you chose Bulbasaur as your Starter Pokemon. If found, it will be at Level 50 and will know the moves Ember, Roar, Fire Spin, and Stomp. Unfortunately, it will attempt to leave on the first turn of the battle. You can attempt to use the move known as Block to prevent it from escaping the battle, but there is a widely known glitch that will prevent you from ever finding Entei again if it uses the move Roar after your Pokemon uses the move Block. Because of this, avoid doing so at all costs.
Since Entei is so very hard to find alone, and because you technically can't stop it from fleeing battle when it is encountered due to a glitch, it would be especially wise to wait to use your Master Ball on it. You could attempt to use an Ultra Ball each time you find it, but it could take a number of frustrating encounters to actually catch Entei when at full health.
Click Here To Learn How To Catch Suicune, The Next Piece Of The Legendary Beast Trio!