Residential fire extinguishers are typically dry chemical (sodium bicarbonate or monoammonium phosphate) and carbon dioxide, and are not hazardous. These may be discharged into the regular trash. However, monoammonium phosphate is an irritant to the respiratory tract and to eyes. So, discharge the unit outside, away from kids or pets, wear goggles and a particulate mask. Then recycle the emptied unit as a plastic or metal container. If you don’t feel comfortable doing that, bring them into the HHW Program.
Other household hazardous waste, see San Mateo County - Household Hazardous Waste