||nsIWindowCreator is a callback interface used by Gecko to create
new browser windows. The application, either Mozilla or an embedding app,
must provide an implementation of the Window Watcher component and
notify the WindowWatcher during application initialization.
||nsIWindowProvider is a callback interface used by Gecko when it needs to
open a new window. This interface can be implemented by Gecko consumers who
wish to provide a custom "new window" of their own (for example by returning
a new tab, an existing window, etc) instead of just having a real new
toplevel window open.