An employee was standing and welding in a scissor lift that was approximately 11'-8" above the finished concrete floor when his clothes ignited and he tried to remove them. He then tried to dismount the scissor lift through the chain gate on the rear of the lift. He attempted to step down the scissor supports of the lift but lost his grip and fell to the ground. He landed on his left wrist and left side. He suffered burns and a broken wrist, requiring hospitalization.