B777 Captain reported a fuel odor and EICAS fuel valve message after engine shutdown at the gate; then was notified to motor the engine by ground maintenance. After dry motoring the engine; the odor dissipated and the Fire Department personnel reported no fire or fuel spill to Captain.

Narrative

After engine shutdown; received status message ENG FUEL VALVE L" and had a very distinct fuel odor in the cockpit.Ground personnel allerted me to an issue on the left side of the aircraft; but the issue wasn't decipherable via hand signals.I opened the cockpit window and a mechanic instructed us to dry motor the left engine - which we did for approximately 2 minutes.During that time; a "fireball of approx. 20 feet" exited out the rear of the left engine and the fuel smell dissapated.My FO (First Officer) requested Crash; Fire and Rescue - who were dispatched onto the scene.While the fuel situation was indeed a safety issue; the purpose of my report revolves around jetbridge movement and potential threat to passengers and ground personnel.During the unfolding events; a CS agent (who was obviously unaware of the fuel issue) moved the jetbridge into position and opened door 2L per normal procedures. Passengers began to deplane (and continued to deplane throughout).My concern is a conflicted one; and I'm not quite sure if the situation was handled appropriately (by me; the ground personnel; or the CS agent). By connecting the jetbridge and opening 2L; it provided an expeditious exit route for passengers should it have been needed - however; due to limited space in the arrivals hallway; it would have filled up quickly; trapping passengers on the jetbridge in the event of expanding fire in the left engine.Choosing to keep the jetbridge clear of the aircraft would have limited escape route to the right side of the aircraft where ground personnel were already approaching the jet. In addition; doors had already been disarmed by flight attendants; complicating any evacuation.As I play the events back; I think we were just plain lucky that the fuel didn't ignite any more than it did and nobody was injured.The ZZZ Crash Fire and Rescue squad arrived at the jet and the lead fire fighter made his way to the cockpit - reporting that no fuel existed on the tarmac and he saw no indication of fire or any other concern."