There are two types of firearms in the game -- semiautomatic and automatic.
Semiautomatic can fire up to one shot for each attack, such as 2 shots/round for a 6th lvl fighter.
Automatics can fire up to one BURST for each attack the character has per round. Most bursts are three-shot, and the second shot in burst hits as well if the modified attack roll was AC+5, and the third if the roll is AC+10. Taking a feat lowers those requirements to AC+3 and AC+6, respectively. Likewise, some large weapons (like the heavy machinegun) fire five-round bursts...
Automatics can also be fired in semiautomatic mode, though, if you wish to conserve ammo...