Final answer:
The thermocouple is the device that will prevent the furnace from firing if it fails, as it is responsible for detecting the pilot light and closing the gas valve if no flame is present.
Step-by-step explanation:
If a furnace has a pilot with a thermocouple, the device that will most likely prevent the furnace from firing if it fails is the thermocouple. The thermocouple is a safety device that detects whether the pilot light is lit by generating a small amount of electricity when it is heated by the pilot. If the thermocouple fails or does not detect heat, it triggers the gas valve to close, preventing gas from reaching the burner and thus stopping the furnace from firing. This ensures that gas is not released into the home when there is no flame present to burn it, preventing potential gas build-up and a hazardous situation.