PyXPCOM allows for communication between Python and XPCOM, such that a Python application can access XPCOM objects, and XPCOM can access any Python class that implements an XPCOM interface. PyXPCOM is actively used in ActiveState Komodo products, for example.
Mozilla defines many external interfaces available to embeddors and component developers. PyXPCOM provides access to these interfaces as Python interfaces. PyXPCOM also contains several classes that provide access to functions for initializing and shutting down XPCOM and Gecko from Python, as well as some XPCOM helper functions.