There are lots of ways of doing this...
you can give your object an instance variable lets say cooloff
then add "cooloff <= 0" to your fire trigger meaning cooloff must be 0 or less to work
then when you shoot add a value depending how long you want to cooloff
on mouse clicked fire bullet
set cooloff to 20 (adjust this number to suit)
then the condition...
cooloff > 0 subtract 1.0*dt from cooloff
sorry bit rushed, hope that helps...