You can not get Mew in Pokemon Fire Red, the only way to get one was going to one of those Nintendo events (like the ones in Toys-R-Us and Gamestop) and using your wireless adapter using your Mystery Gift option.
Sadly Nintendo has stopped all events on GBA generation type of games. You can however use a code using Gameshark, which is in my opinion, the only way to get these rare Pokemon.
This is the way to get Mew, the Must Be On code MUST BE ON, while you apply the other codes, no exceptions! Choose the level you want also, do not put both level codes!
- [M] Must Be On - July 01, 2005
- Level 50 - June 10, 2005
- Level 100 - June 10, 2005
- Mew - June 10, 2005