An Emirates Boeing 777-300, registration A6-ECW performing flight EK-237 from Dubai (United Arab Emirates) to Boston,MA (USA) with 374 people, had safely and normally landed on runway 04R and was taxiing across runway 04L, 15L holding short of runway 15R when the crew reported a fire indication for the left hand engine. Tower advised there were no visible flames and queried whether the crew wanted to hold right there or continue taxi, the crew indicated they wanted to cross the runway, tower instructed to turn left onto runway 33L and advised emergency services were on their way. While taxiing along runway 33L tower advised there were flames visible from the left hand engine. When the emergency services arrived they couldn't see any fire, tower advised flames were no longer visible, they had been visible from the back of the engine. The crew advised they never had a fire indication, but could see some flames on the camera, which stopped after the engine was shut down. Emergency service subsequently reported they were seeing an active fuel leak, the crew shut down the aircraft and reported they had a strong smell of fuel on board. The passengers were kept on board until the aircraft could be towed to the apron.