Actually, I used firearms in RQ3 already, and they worked extremely well in historical situations. They are exceptional support weapons, able to win a fight with a simple hit, while a bow or sling need a critical to disable the same foe, but the long reload time and the propension to jam/explode mean that you cannot rely solely on them .
The trick is to use them in the historical way. Cortez conquered Mexico with his superior weaponry, but when he came back none (yes, none) of his muskets was still working: all had been destroyed in combat. In the end, the Spanish had to fight with swords and pikes, much more reliable weapons that were still superior to Aztec weaponsry. When I ran my conquistador game years ago, I used the musket stats for Land of Ninja (3d6 damage, a big hit in RQ3) and ruled that a 96-00 roll meant the weapon was rendered useless (exploded on 00), requiring repairs that were not possible while in the wilds. And it worked just fine: the muskets often made the difference and they won at least one major battle with a big monster, but by the end of the scenario most of them had gone "boom" and the players were all fighting with swords.