there is only two ways to get celibi on pokemon fire red the first is going to a nintendo special event but nintendo no longer do special events for pokemon fire red the second is cheting with a game shark or action replay but this is cheating so consequently mewtwo is the hardest pokemon to catch on pokemon fire red
Don't use it on Mewtwo, you can catch him with Ultra balls. When I did Fire Red, I had most of the Pokemon and I never used my Master ball. Apparently you need to save it for Celebi, who is supposedly a secret Pokemon in FR. Though I don't know if that's true though.
The explanation just posted from me is from my son who likes pokemon a little bit ahem
my son says always use it on mew2 found in cerulian cave