We all use the term buff to indicate someone who has a deep interest in some subject. A Cinema-buff is take a lot of interest in cinema, and then there is the sports-buff, the car-buff and numerous other buffaloes out there.
It was Benjamin Franklin who set up the first firefighting force in Philadelphia in 1736 under the guise of the Union Fire Company. A lot of the buildings at the time were built with wood, and fires were a frequent occurrence. A large fire could consume many houses. He even instituted the first property insurance through which the firemen were paid to ensure fires were put out quickly.
By the 19th century, the science of firefighting had improved, and so had the gear. Firefighters used to wear coats made with buffalo-hide since they were often working in the night in freezing weather. This led to the firefighters being called buffs.
Every time a fire broke out, there would be a group of young boys who would rush to witness the fire being put out. These boys began to be referred to as fire-buffs. Like that the meaning of the word transformed and came to possess the form in which it is used today to describe any person who is deeply interested in any area.