Treat firearms with care. After all, this is the next best weapon technology that will replace most melee and ranged weapons in the field of battle, including the lance and crossbow.
Don't let them have Craft (Gunsmithing) skill so he can make bullets and the required gunpowder. Leave that to a NPC expert class (or better yet, create a NPC Gunsmith class with gunsmithing ability as its class feature).
While the PC party may have the advantage, once in a great while throw in an equalizer: let the enemies use firearms, too. Of course, don't place them where they can be reached or the party may score more firearms as spoils.