As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING CLIMB, THE NR 3 ENGINE FIRE WARNING ACTIVATED. AIRCRAFT RETURNED TO NRT AND LANDED WITHOUT INCIDENT. REPLACED THE IGNITER PLUG BOSS, HIGH STAGE VALVE, AND HIGH STAGE CONTROLLER. OPERATIONAL CHECK GOOD. EXACT FAULT CAUSING DISCREPANCY TO BE DETERMINED.