What should be understood by "refrigerate until firm, 2 hours or overnight"? The cookie recipe asks for:

1½ cups plus 1 tablespoon all-purpose flour  
1¼ teaspoons ground ginger  
1 teaspoon ground cinnamon  
¼ teaspoon ground cloves  
¼ teaspoon freshly grated nutmeg  
1 tablespoon unsweetened Dutch-process cocoa powder  
¼ pound (1 stick) unsalted butter, room temperature  
1 tablespoon freshly grated peeled ginger  
½ cup packed dark brown sugar  
½ cup unsulfured molasses  
1 teaspoon baking soda  
1½ teaspoons boiling water  
7 ounces best-quality semisweet chocolate, cut into ¼-inch chunks  
¼ cup granulated sugar  


then, it asks us to combine all these ingredients and 


  ; turn onto plastic wrap. Pat out to a 1-inch thickness; seal with wrap. Refrigerate until firm, 2 hours or overnight.


Questions:


How do you define "firm" here? Something which is not melting, or something which we'll have to crack with our hands?
There is a great difference between 2 hours and overnight. So, what temperature should be set of the fridge that'll result in fast firmness in less time? Will that be counter productive? Can I use the freezer?
When I've seen that step in cookie recipes, generally there's no way you can over-firm in the fridge; if you're doing all the steps on the same day, it'll take two hours to set, but if you're trying to prep the night before, you can break for the night at this step.

Generally you'll have a wet, sticky, soft dough you can't cut into shapes, and after a trip to the fridge, it'll be firm enough to hold a shape when you go at it with a cookie-cutter (or roll it into a pinwheel and slice, or whatever shaping is happening). It's basically letting the butter go back from room-temperature-butter consistency to fridge-butter consistency.