Wikipedia
XPCOM
Cross Platform Component Object Model (XPCOM) is a cross-platform component model from Mozilla. It is similar to Microsoft Component Object Model (COM) and Common Object Request Broker Architecture (CORBA). It features multiple language bindings and interface description language (IDL) descriptions; thus programmers can plug their custom functions into the framework and connect it with other components.
In August 2015, Mozilla announced that the use of XPCOM for creating Firefox add-ons would be deprecated in the future in favour of WebExtensions. Pale Moon, a fork of Firefox, will continue to support XPCOM indefinitely. __NOTOC__