Xpcom

Although most components available from XPCOM are written in C++, the XPConnect/XPCOM pairing can also accommodate other languages.

Source: wiktionary

XPCOM is entirely internal to the Mozilla Platform. XPConnect is the glue that turns XPCOM-compliant objects and their interfaces into JavaScript host objects.

Source: wiktionary

XPCOM enables component software development, a paradigm that is heavily leveraged in the Mozilla product line. DLLs that implement the required external functions and classes defined for this purpose by XPCOM can be registered as components with a Mozilla-based project, and applications can then uses XPCOM to query these components for classes and interfaces, and then make calls against the interfaces to obtain the services provided by the component.

Source: wiktionary

These XPCOM interfaces are the starting point for all XUL elements.

Source: wiktionary