Federal Firefighter Flexibility and Fairness Act This bill provides that any hours worked by a firefighter under a qualified trade-of-time arrangement shall be disregarded for purposes of determining eligibility for, or the amount of, any overtime pay. The bill defines "qualified trade-of-time arrangement" as an arrangement under which two firefighters who are employed by the same agency agree, solely at their option and with the approval of their employing agency, to substitute for one another during scheduled work hours in the performance of work in the same capacity.