I actually think that casualty rates generally declined with the invention of firearms. While the front ranks would certainly take extremely high casualties during an exchange of volleys, the likelihood of an individual soldier finding himself in that situation tended to decline, thus reducing the overall probability of being wounded or killed. This general decline was most signficantly due to the fact that the ease of use of gunpowder weapons (along with other technological advances) allowed nations to start fielding much larger forces. With exponentially more people fighting, your odds of being unlucky enough to be caught in the front lines at the exact moment of a heavy attack were minimal.

I have no statistics to prove this off the top of my head, but I bet we can dig some up.