Components.interfacesByID is a read-only array of classes indexed by IID.
Introduction
Components.interfacesByID is a read-only array of objects which implement the nsIJSIID interface. Each object represents one of the XPCOM interfaces -- that some component or another might or might not implement. It reflects only those interfaces which have been designated in their .idl description as [scriptable], i.e. the interfaces which XPConnect is capable of reflecting into JavaScript.
Components.interfacesByID is exactly like Components.interfaces except that the elements are indexed by the canonical form of their IID.
Usage
You can retrieve an interface object using:
var iface = Components.interfacesByID["{aa610f20-a889-11d3-8c81-000064657374}"];