Mario's Fireball uses a Sub Routine for its damage.
If you're using PSA, open up your FitMario.pac, go the the Sub Routines tab, and enter this Offset into the text box: 17108. Press Go afterward.
You should see the following coding:
Special Offensive Collision: Id=0, Bone=0, Damage=5, ShieldDamage=0, Direction=361, BaseKnockback=30, WeightKnockback=0, KnockbackGrowth=25, Size=2.4, Z Offset=0, Y Offset=0, X Offset=0, TripRate=0%, HitlagMultiplier=x0.3, SDIMultiplier=x1, Flags=402853893
Asynchronous Timer: frames=5
Special Offensive Collision: Id=0, Bone=0, Damage=5, ShieldDamage=0, Direction=361, BaseKnockback=22, WeightKnockback=0, KnockbackGrowth=20, Size=2.2, Z Offset=0, Y Offset=0, X Offset=0, TripRate=0%, HitlagMultiplier=x0.3, SDIMultiplier=x1, Flags=402853893
Asynchronous Timer: frames=30
Special Offensive Collision: Id=0, Bone=0, Damage=5, ShieldDamage=0, Direction=361, BaseKnockback=11, WeightKnockback=0, KnockbackGrowth=15, Size=2, Z Offset=0, Y Offset=0, X Offset=0, TripRate=0%, HitlagMultiplier=x0.3, SDIMultiplier=x1, Flags=402853893