A charter flight arrived and deplaned without incident and with no indication of smoke, fumes, or heat. Company maintenance personnel had placed damaged passenger seatbacks (incorporating solid-state chemical oxygen generators) in the forward cargo compartment with seat covers and oil. A company mechanic examined the seatbacks to find a serviceable unit. He found a loose oxygen canister and handled it improperly by its oxygen hose. Such handling can release the firing pin, fire the percussion cap, trigger a chemical reaction, and generate oxygen. This can generate up to 430 degrees of heat outside the canister. A fire began in the forward cargo compartment in the vicinity of where an oxygen canister was found (later) with a dented striker plate. The seat covers ignited, fire burned through the cabin floor, and subsequently, it spread throughout the entire cabin. Company maintenance personnel did not know that oxygen generators, carried as company supplies, were hazardous materials. The mechanic was not familiar with the repair or installation of the passenger supplemental oxygen equipment.