If the correct quality fuel is burnt in the right manner, the glass should stay relatively clean. The air wash which passes down the inside of the door will scrub off any deposits during the burn cycle. If the glass is becoming dirty then scrunch two pieces of damp newspaper, dip one in cold fire ashes and rub over the inside of glass, use the other to rub over the glass to clean off the dirt. Do this in the morning before rekindling the fire as the glass will be cool enough at this time. To help keep the glass clear and clean if it is becoming dirty then get into the habit of cleaning it regularly as this will maintain the glass and prevent ashes from being fused onto the glass due to intense heat in the firebox.