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.

XPCOM is one of the main things making the Mozilla application environment an actual framework. It is a development environment that provides the following features for the cross-platform software developer:

This component object model makes virtually all of the functionality of Gecko available as a series of components, or reusable cross-platform libraries, that can be accessed from the web browser or scripted from any Mozilla application. Applications that must access the various Mozilla XPCOM libraries (networking, security, DOM, etc.) use a special layer of XPCOM called XPConnect, which reflects the library interfaces into JavaScript, or other languages. XPConnect glues the front end to the C++ or C programming language based components in XPCOM, and it can be extended to include scripting support for other languages: PyXPCOM already offers support for Python. PerlConnect and plXPCOM provide support for Perl, and there are efforts underway to add Common Language Infrastructure (CLI) and Ruby language support for XPConnect.

...
Wikipedia