There are several types of fire extinguishers, some which contain hazardous chemicals. The following companies can recharge, test and properly dispose of fire extinguishers:
Gorham Fire Appliance Company, 288 Willard St., Quincy, 617-472-5785
R E Lyons and Sons Fire Equipment, 197 VFW Dr., Rockland, 781-878-7599
If you have a red tank containing CO2 (carbon dioxide), you may discharge it outdoors, unscrew and remove the head or drill a hole in the cylinder, and recycle with scrap metal.