B777-200 First Officer reported diverting to an alternate airport after experiencing a Master Warning for FIRE FWD CARGO.

Narrative

I was the relief pilot of the flight from ZZZ to ZZZZ1 on Day 0; and assumed the role of Pilot Flying once I returned to the flight deck. While in cruise about 15 minutes from Waypoint 1 a FIRE FWD CARGO Master Warning and fire bell activated. I continued flying the aircraft after announcing 'My Aircraft' while the First Officer (FO) ran the Checklists. We sidestepped 5nm to the left of our route and [priority handling was requested]. We initiated a left turn towards ZZZZ after contacting company and ZZZ [Center]. After the aircraft was secured; weather was obtained and the pertinent people notified; I called the Captain whom was on their rest period during the event; by this time the FO had assumed Pilot Flying duties and I briefed the Captain on the situation; weather and plan. They then occupied the left seat and Flight Attendants were briefed about our situation and prepared the cabin for a precautionary landing at ZZZZ. The Fire Warning message was present during the majority of the event after the completion of the checklist. We came to the conclusion that we needed to jettison fuel since we were going to be overweight and we coordinated and attempted to jettison fuel twice but the system failed to activate; We then decided to proceed with an overweight landing. While FO flew the airplane; I ran landing assessments and provided latest weather for ZZZZ. Our landing weight was approximately 481;000. A normal approach and landing was performed on Runway XX and the aircraft was inspected by Airport Rescue and Firefighting (ARFF). While taxiing to a parking location the FIRE warning returned; ARFF escorted the aircraft and positioned themselves around the aircraft in case they were needed. We deplaned passengers and crew safely after everything was secured and coordinated with airport personnel.