A Boston firefighter braved frigid cold this morning to rescue a dog who fell through Boston Harbor’s ice into the freezing cold water at Pleasure Bay. Sylvie, a 13-year-old Husky, was walking with her owner when she ran across the ice near Castle Island and fell through. After receiving the call, firefighter Sean P. Coyle of Ladder 19, a seven-year vet, donned the survival suit and rode out onto the ice using a sled-type piece of equipment called the “stokes basket.”