I was looking through the Pathfiner feat selection, and noticed Prone Shooter.
The PFSRD text helpfully links to the Prone condition, which describes the penalties and benefits associated with being Prone.

So what penalties is Prone Shooter supposed to negate? I'm a little confused right now.

It's due for errata I think.

I'm guessing the feat itself is the error and there was never supposed to be a penalty to attack for crossbows and firearms (as makes sense).

Does it allow you to use firearms while prone? The Prone condition makes mention of crossbows being usable while prone, but not firearms.

Apparently, when the creator of the feat wrote it, it gave some small bonuses to firing from a prone position. The editors decided that was too powerful, and simply had it negate the penalties, somehow forgetting that there are no penalties to firing a crossbow or firearm from prone.

EDIT: Ultimate Combat clarifies that firearms can be used while prone. So the feat still does nothing.

If I were redesigning the feat, I'd allow a small bonus if you took a full round action to line up a single shot.

Yeah, make it a sniper feat. That's the only thing that can save it.

I guess it depends on the firearm. I don't recall musketmen firing prone, but a marine with a post american civil war rifle would certainly do it (mostly to do with loading). Musketmen usually stoop to load or kneel to fire and those in the rear can fire while standing.

That said, I dunno how a crossbow man could manage to load while laying down (apparently loaded crossbows can even jostle and misfire while moving; good way to balance this out maybe by making them better against armor or do more damage).