There is nothing wrong with increasing one’s confidence, will or belief in themselves.
However, this should be subject to the laws of nature, physics, thermodynamics, etc.......
Remember, damage from heat requires direct contact and sufficient quantity of hot materials. A properly conducted firewalk is on long-burned coals with an ash layer, which insulates the actually hot part of the coal from the skin when you walk on it.
Detailed explanation: http://entertainment.howstuffworks.com/arts/circus-arts/firewalking.htm