JavaXPCOM allows for communication between Java and XPCOM, such that a Java application can access XPCOM objects, and XPCOM can access any Java class that implements an XPCOM interface. JavaXPCOM is not actively maintained.
Note: JavaXPCOM was included in XULRunner up through version 1.9.2. It has been removed in XULRunner 2, and is not actively maintained.
XULRunner ships with the JavaXPCOM component, which allows Java code to interact with XPCOM objects. As you will see in this article, working with XPCOM objects in Java is not that much different than doing so in C++.