You can't
There is no Mt. Sliver on FireRed
Firered don't have a "solar cave"
you dont use it it only unlocks a cave
You can get Mewtwo in the Cerulean Cave.
altering cave,
in the land cave.
bartonvilleType your answer here...
Cerulean Cave
mewtwo is in mystery cave :D
icefall cave firered only
icefall cave
in the yellow area