Can a fire department or EMS employee volunteer for the department during their off-duty hours?

NO! Under the FLSA, nonexempt employees may not do the same work as volunteers that they do as employees. If firefighters or paramedics show up at a call when they are not scheduled to work, then the employer must either send them home or pay them for that time. Firefighters and EMS workers may volunteer for other jurisdictions, and local governments may use firefighters and EMS workers employed by other jurisdictions as volunteers, but a person cannot be both an employee and a volunteer in the same capacity for the same employer.