As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ON TAXI OUT, APU FIRE WARNING SOUNDED. CREW PERFORMED APU SHUTDOWN PROCEDURES AND FIRE BOTTLE DISCHARGED. AIRCRAFT RETURNED TO GATE. INSPECTED WITH NO EVIDENCE OF FIRE OR OVERHEAT CONDITION. REPLACED FIRE BOTTLE PER BOEING M/M, CONTINUED PER MEL 26-6.