The glue of the box cam be loosened with a warm iron carefully applied to the outside of the box, and resealed the same way after you've rehelmeted the fireman.
I'd work from the bottom of the box, and hold the box tightly shut while the glue cools down again, by propping it with books or somesuch.
A hot glue gun can be used to refresh the glue seal if needs be, and can fix a number of other Playmobil problems too.