1. Wash with ammonia **Recommended** - Make sure fire place is cool to the touch. Fill a spray bottle with a slightly diluted mixture of ammonia and water. The water helps keep the ammonia from evaporating on the glass and makes it easier to use. Some also recommend using some vinegar in the mixture, although this is not necessary.
Grab some paper towels and spray the glass. Start wiping the black or foggy glass clean. It will take a couple of times to get a clear window depending on how coated your glass is. If there are some unusually difficult spots let the solution sit and react.
Also take your ammonia and water soaked paper towel and dunk it in the ashes in the bottom of the stove. Using this to scour the glass as the ash will aid in the chemical cleaning of the glass. Wipe clean with a dry cloth.
2. Use a Commercial Cleaner- There are a variety of products on the market that can accomplish the same as ammonia. Follow directions of whatever you choose, and make sure they do not void your stoves warranty.