Only firefighters and security guards who are members of the West Virginia National Guard may be employed by the Adjutant General as firefighters and security guards: Provided, That any person employed as a firefighter on the effective date of this section who is not a member of the West Virginia air National Guard may continue to be employed as a firefighter: Provided, however, That no person who is not employed on the effective date of this section as a firefighter and who is not a member of the West Virginia air National Guard may be employed as a firefighter for the West Virginia air National Guard.