(a) A firefighter who has not completed 20 years of creditable service and is receiving a disability pension under this Article whose disability continues for a period which when added to his or her period of active service equals 20 years may, if age 50 or over, elect to retire from the fire service by submitting written application to the board. A firefighter exercising such option shall be entitled to continue to receive a retirement pension equal in amount to the disability pension he or she was entitled to as a disabled firefighter on the date he or she was removed from the municipality’s payroll for disability. A firefighter electing to exercise such option shall be entitled to the automatic increase in pension provided under subsection (a) of Section 4-109.1.
     (b) A firefighter who is receiving a disability pension under this Article who has sufficient creditable service to qualify for a retirement pension and is age 50 or more may elect to permanently retire from the fire service at any time by submitting written application to the board. The salary to be used in the determination of such firefighter’s pension shall be based on the salary attached to the rank held by the firefighter in the fire service at the date of the election to retire. All other conditions in the computation of the pension shall be based upon the provisions of Section 4-109 which were applicable to the firefighter while he or she was in active service as an employee. A firefighter electing to exercise such option shall be entitled to the automatic increase in pension provided under subsection (a) of Section 4-109.1.