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.
The most prominent usage of XPCOM is within the Firefox web browser. Many of its internal components interact via XPCOM interfaces. Furthermore, Firefox used to allow add-ons extensive XPCOM access, but this was removed in 2017 and replaced with the less-permissive WebExtensions API. (Three forks of Firefox still support the legacy add-on capability: Pale Moon, Basilisk, Waterfox.)
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:
The flexibility to reuse the XPCOM components from the Gecko library and develop new components that run on different platforms facilitates rapid application development and results in an application that is more productive and easier to maintain. The networking library, for example, is a set of XPCOM components that can be accessed and used by any Mozilla application. File I/O, security, password management, and profiles are also separate XPCOM components that programmers can use in their own application development.
XPCOM adds a lot of code for marshalling objects, and in the Netscape era XPCOM was overused for internal interfaces where it wasn't truly necessary, resulting in software bloat. This was a key reason why in 2001 Apple forked KHTML, not Gecko, to create the WebKit engine for its Safari browser.