Racing over drought-parched terrain and fanned by winds that reached 80 k.p.h., a series of fierce fires ravaged large areas of the French Mediterranean coast and Corsica last week. Thousands of soldiers and fire fighters as well as scores of water trucks and aerial tankers were mobilized as flames destroyed more than a score of homes and blackened more than 23,000 hectares of bush and forest in the Bouches du- Rhone, Var and Alpes-Maritimes regions. A fire fighter was killed when an explloding tire caused his truck to roll over.