Even though Pokemon FireRed takes place in the Kanto Region and Mew is a Pokemon found within the Kanto Pokedex, it actually cannot be found in the Wild within the game. Believe it or not, there was never even a Pokemon Event to obtain the Mythical Pokemon Mew when Pokemon FireRed was first released and when the Nintendo GameBoy Advance was still the main Nintendo Handheld Device. Because of this, the only way to obtain Mew back in the day was to Trade with someone playing Pokemon Emerald who had Mew. These days, the only way to obtain Mew is by using Cheat Code Device.
If playing on an actual Game Boy Advance, you will need either a Action Replay Device or a Codebreaker Device to be inserted into your Game Boy Advance in order to input a cheat code to obtain Mew. If you are playing on an online emulator, you simply need to download one that supports these types of codes. If using a Codebreaker Device (or are playing on an emulator that supports their given codes), you will need to enter the code "000014D1 000A" on the first line, "1003DAE6 0007" on the second line, and "83007CEE 0097" on the third. This will cause Mew to appear in the Wild within Pokemon FireRed.
Looking to obtain Mew by using an Action Replay Device instead? Then check out our Action Replay Codes for doing so by Clicking Here or by Clicking Here!
Doesn't work. It said that it seems to be from a different game.