So Rapid Reload works on both Crossbows and Firearms. What about Crossbow Mastery? Can it work on a Firearm or is that completely up to the GM? I can't really find anything that says definitely one way or the other.
Crossbow Mastery only affects crossbows. It's very clear that it only affects crossbows.
You can load crossbows with blinding speed and even fire them in melee with little fear of reprisal.
Prerequisites: Dex 15, Point-Blank Shot, Rapid Reload, Rapid Shot.
Benefit: The time required for you to reload any type of crossbow is reduced to a free action, regardless of the type of crossbow used. You can fire a crossbow as many times in a full attack action as you could attack if you were using a bow. Reloading a crossbow for the type of crossbow you chose when you took Rapid Reload no longer provokes attacks of opportunity.
Special: Starting at 6th level, a ranger with the archery combat style may select Crossbow Mastery as a combat style feat.