Looking more about the Arsonist, his role as Serial Killer is this (from that mafia wikia)
The Serial Killer is a lone killer without allegiance. The Serial Killer kills once per night, and his goal is to be the sole survivor. Frequently they are not allowed by the Game Moderator to forgo their night kill.
Serial Killer is generally considered to be one of the hardest roles to win with. To compensate for this, some mods have been known to give Serial Killers extra abilities, such as making them immune to either some or all night kills, or having them show up as innocent to Cop investigation.
I'm definitely in for the Arsonist/Firefighter roles, they sound very very interesting. Though, of course, we need more people here, if we can only get 10 this won't be as much fun