I know this has been brought up many of times. Crossbows and firearms while prone rules are in my opinion far from what is acceptable. Here are my rules in my campaign:
Firing Crossbow or Firearm while prone: +2 to Attack Roll.
Reason: Anyone with the slightest of weapons training knows that it's vastly easier to hit the 300 lying down than standing up.
Firing Crossbow or Firearm while prone with the feat: +4 to attack roll.
Reason: Like most feats, you basically are specialized at firing while prone.
Here's a custom rule that I also use:
Supported Firing Position: +1 to Attack roll
Reason: Ranged can suck sometimes when firing into Melee, this is something that I feel helps balance out ranged/melee. A supported firing position is when you prop your weapon onto something like a table, chair, fence, wall, what have you, for stability.
Anyone have any thoughts on these rules?