You need to make sure that you seal your prehung exterior french doors against the cold this year. Even a small crack can be enough to make your home uncomfortable, as the cold air will get in and the warm air will escape. This will create a draft that will be felt through the home. If the builders have not done a good enough job of sealing the cracks around the doors, you may have to do a little bit of extra work to ensure that this is done.
This is mostly a problem with exterior French doors since they open to the outside. If they are not tight up against the floor, a draft can get through. You will need to purchase rubber strips to use in sealing the door. These can be attached to the bottom edge of the door, and they will reach all the way down to the floorboards. Since they are rubber, they will be very flexible, and this is crucial to the process. It means that they can bend and fit exactly to the floorboards, cutting off all drafts at the source. Since they are flexible, you will still be able to swing the door open or closed with ease.
While blocking drafts is not as important with interior pine French doors, it is still something that you may want to consider. If the builders who hung the doors left any cracks between the trim and the wall, you can fill them with silicon caulking. This dries clear so that it will be invisible. This tactic is generally used on doors that open to a garage or an uninsulated room. The air in these places will be colder than the air in the house, so drafts are still possible even though the doors are inside.