The incident happened while the DFDS Seaways ferry, which was due to set sail for the French port of Calais, was still in dock, Dover port authorities said.
"All ferry passengers have been safely disembarked," the Port of Dover said in a statement.
Three firefighters suffered "flash burns" and a number of crew members were also hurt when the fire broke out in the vessel's engine room, Kent Fire and Rescue Service said.
Paramedics said they treated a total of ten people, eight of whom were taken to hospital with burns to the face, arms and hands.