"Operation Firebird" is the name given to the heroes' mission to help the lost souls of the Underworld complete their unfinished business and move on.
After the Third Curse
When Emma, Regina, Mary Margaret, David, Henry, Robin and Gold arrive in the Underworld, they come to realize after Prince Henry's ascension to Mount Olympus that all the lost souls down there have resolvable business that could help them move on. Gold refuses to help in this new mission, while Henry takes it upon himself to name it "Operation Firebird". ("Souls of the Departed")