How Do Get Celebi In Pokemon Fire Red?

I heard that you can also get mew, dexoxes, or jarachi this way but only one
so here's how you do it. After you become the champion and unlocked the national Pokedex, and have your starter at lv 75 or above and become the champion. Go and talk to your mom 42 times and she will say that she is lonely now that you and your dad are gone. She will then ask you to go and find your father, if she doesn't ask this just continue with the following steps. First, walk to Indigo Plateau, not fly or ride your bike, just walk, and then talk to nurse joy. She will say she remembers your father and that he was dear friends with lt. Surge. Fly to Vermillion City and stand in front of the gym for five minutes, but more is better. Then go inside and talk to surge. He tells your father left to find the pokemon of legends. Go to Seafoam islands and there is your dad. Talk to him and he will battle him. He has a lv. 85 Jirachi, a lv.85 Deoxys, a level 90 mew and a lv. 90 Celebi. Once you win he will reward you with any one of his pokemon.