Pikachu is the face of the Pokemon Franchise, being introduced way back when in the 1st Generation of Pokemon. Since Pokemon FireRed is a remake of Pokemon Red and thus is based upon the 1st Generation of Pokemon, Pikachu can be found naturally in the Wild within the game. The first location a player can find Pikachu is early in the game in Viridian Forest, but it's pretty darn hard to find (only appearing about 5% of the time in a Wild Pokemon Encounter). Pikachu can also be found later in the game in the Power Plant, being easier to find this time around (appearing in 25% of Wild Pokemon Encounters).
Unfortunately, these are the only known ways to find Pikachu with the structure of the game. Of course, Pikachu is available by Trade should you connect with someone via the Wireless Adapter. You can even obtain Pikachu via a cheat code with either an Action Replay Device or Code Breaker Device.
Looking to see how to obtain the iconic Charizard? Click Here To Learn How!