2002 Ford Mustang water leakage

Definately water- no smell or color to it. Car was driven until engine was warmed up approx. 40 minutes. It was a warm day 75 degrees, and kind of humid. I did not have the AC on, just the fan set on cool to circulate the air. About an hour after I parked in my driveway, I noticed a puddle under the front end. With the hood up I restarted the car and my husband saw water at first gush and quickly turn to a dribble from a round opening in the firewall. He thinks its the AC drain opening. As soon as I switched to heat it stopped. Today I drove the car after rechecking the above with no repeat of the problem, the temp guage stayed in the normal range and no water leakage. No debris whatsoever around car.

Thank -you Paul! You reassured me. Sorry if I seem like a dumb woman but this is the first time this has happened in the 3 years I've had my car! And of course Something" leaking" puts me into panic mode since my car is 6+ years old! Thanks again!