You shouldn't need to put your no bake cookies in the fridge to firm up. They will do that on their own within about 20-30 minutes of dropping the cookies onto a cookie sheet as the oats soak up additional moisture and the chocolate peanut butter mixture cools. Sticking the still-warm cookies into the fridge to speed up the process can actually affect the cookies negatively by resulting in setting the chocolate portion before the oats have a chance to absorb moisture, which changes the end texture of the cookies. BUT, if for some reason your cookies don't set up after 20-30 minutes, you can stick them in the fridge at that point and everything will turn out fine. (And if that happens, chances are you probably didn't boil the sugar mixture quite long enough. But don't worry, that's not going to happen to you.)