I still land on the "exposition exists for a reason" square. Jayne tells us Vera needs air around her to work then that's the way it is until proven otherwise. Which I guess you could argue the decompression of the suit does but, then, we have to fall on the "air in the suit" remaining the same regardless of the reality of what would happen.
The FF guns did make a whirring or odd sound when used which does suggest something to them that means they work differently than the guns we have today. That and it's the future, so.... I still land on just because it looks like a present-day gun doesn't mean it IS and works like one.
The movie "Santa Claus vs. The Martians" features a Martian weapon that in actuality was a toy that "shot" a blast of compressed air. Yet in the movie the weapon was a standard Martian stun-gun. The logic some seem to be taking seems to be that "that gun shouldn't stun people because it only shoots compressed air!"
What it looks like is different than what it IS.