According to Wikipedia:
Walking on fire has existed for several thousand years, with records dating back to 1200 B.C. Cultures across the globe, from Greece to China, used firewalking for rites of healing, initiation, and faith. Firewalking became popular in America during the 1970s when author Tolly Burkan began a campaign to demystify the practice. He offered evening firewalking courses that were open to anyone in the general public.
Where firewalking used to belong to the realm of mystics and magic it suddenly became within the reach of anyone with the mental ability to change their perception. Those who have experienced it find it liberating and empowering to bear witness to the power of mind over matter, while those who witness it cannot help but stand in awe of a feat that defy logic in direct opposition to our instinct to survive.
Even though it is a practice that has been around since the time of Genesis, the actual physics that explain why people do not get burned during the experience can only be found in some very recent scientific discoveries about the nature of water. And while you may think that the molecular properties of something as common and important like water would be old news, fact is we have only recently begun to understand its nature.
Water has a highly crystallized structure, even when it is in a fluid state. , and this affinity to form crystal lattices is one of the main reasons why proteins all have a specific three dimensional structure. It is the structural variety of proteins that give them the ability to combine and interact with other proteins in what can almost be described as a molecular dance. A dance that expresses itself in the infinite variety of life that share our planet.
When protein molecules are exposed to increasing temperature, it is the collapse of the supporting water molecules that causes proteins to lose their structure, but water is also known to exhibit a period of latency between the time that it reaches boiling temperature and the time it eventually begins to boil. It is this natural ability of water that provide the scientific explanation why it is physically possible to walk on fire.
Thus the belief that the fire walkers express as part of the ritual of walking on fire seem to be translated to a force that allow water molecules to maintain their crystalline structure under the extreme duress of temperatures that can be as high as a thousand degrees centigrade. It also supports various other experimental findings that intention can influence the crystal structure of water, and bears testament to the incredible ability of human consciousness and the power of intention.