The mirror has no effect on spells which affect more than one person (such as Fire Storm).
Full info about magic mirror :
Spells cast on a subject protected by this spell are reflected back upon the caster of that spell. When a spell is reflected the very internals of the spell is also affected, so that not only do the caster become the target, but the subject of the mirror also assumes the role of caster of the reflected spell.
The Magic Mirror protects only during the turn in which it was cast The protection includes spells like Magic Missile and Lightning Bolt but does not include attacks by monsters or stabs from wizards. In case of multiple mirrors in a melee, a Magic Mirror will not reflect any spells originating from the wizard that the mirror protects, because the magic of the spell then already will be present inside the mirror.
The mirror is countered totally if either a Counter Spell or Dispel Magic is cast on the subject in the same turn as the mirror. The mirror has no effect on spells which affect more than one person (such as Fire Storm). Two mirrors cast at one subject simultaneously combine to form a single mirror.