There are a couple of ways, either an instance variable for the amount of frames left until you can shoot, a timer, or an instance variable which sets the time with a later event to check if [time] is greater than the variable + something.
1. I fire, it sets player.firerate to its default, 100.
2. Every frame, decrease (firerate) by 1.
3. When you fire, it checks that player.firerate ≦ 0
Not the best way, but it gets the job done.
Here's a capx.