Components.classesByID is a read-only object whose properties are classes indexed by CID.
Introduction
Components.classesByID is a read-only object whose properties implement the nsIJSCID interface. Each property represents one of the classes of XPCOM components that can be constructed or accessed as an XPCOM service.
Components.classesByID is exactly like Components.classes except that the elements are indexed by the canonical form of their CID, and does not only represent the component classes that have been registered with the component manager using a ContractID, but also those registered using a plain CID.
Usage
In order to retrieve the object for a given CID, you can use that CID as an index with Components.classesByID as follows:
var clazz = Components.classesByID["{ed132c20-eed1-11d2-baa4-00805f8a5dd7}"];