Out of all the Poke Balls a player can obtain within Pokemon FireRed, the Master Ball is the best available. This very special Poke Ball has a guaranteed capture rate, meaning you will catch whatever Pokemon you use your Master Ball on no matter what. Because a Master Ball guarantees a Pokemon Capture, it comes as no surprise that they are extremely rare. Believe it or not, you will only be able to obtain one Master Ball during the Main Story! That's it!
In order to obtain thw Master Ball, all you must do is follow the Main Story closely. After saving Mr Fuji from Pokemon Tower and obtain the Tea from the Elderly Woman within the Pokemon Mansion in Celadon City, you will earn access to the previous inaccessible Saffron City. Upon your arrival, you will find that Team Rocket has taken over the city and is currently causing trouble in the Silph Co. Building within Saffron City. After defeating Team Rocket at Silph Co., the head of the company will gift you with the Master Ball for your help.