(a)(1) Any person employed as a firefighter or enrolled as a volunteer firefighter is eligible to make application to the board for membership in the fund.

(2) Upon becoming a member of the fund, a firefighter or volunteer firefighter shall receive credit only from the date of his or her becoming a member of the fund.

(b) After April 1, 1989, no person who is a member of the Peace Officers' Annuity and Benefit Fund shall be eligible for membership in the fund by virtue of any employment in or appointment to a position the duties of which qualify such person for membership in the Peace Officers' Annuity and Benefit Fund.