Not sure about the veracity of this article, but it suggests the following:
Plasma displays are normally specified to operate within a temperature range of 32 - 100 degrees Fahrenheit (0 - 40 degrees Celsius). The actual supported ambient operating temperature range varies from model to model, so please check with the accompanying product literature. This information is normally included as part of the product technical specifications and covers details for temperature and humidity ranges applicable to both operating and storage conditions.
Burning a fire with the plasma over the fireplace while your plasma TV is off, will not harm your flat-panel TV. This being so as long as the ambient temperature is within the storage temperature range for your plasma TV; the latter is typically from 15 to 120 degrees Fahrenheit (approx. -10 to 50 degrees Celsius).