XPConnect

XPConnect is a bridge between JavaScript and XPCOM. With XPConnect, you can use XPCOM components from JavaScript code, and interact with JavaScript objects from within XPCOM components. XPConnect is part of Firefox and is actively used in XUL applications.

Documentation

Architecture basics: XPConnect, JavaScript, XPCOM, XUL...
Using components: How to talk with XPCOM components.
XPConnect and XPIDL FAQ: A FAQ about using XPConnect and XPIDL. This page needs to migrate to MDN.
Wrappers: What sorts of wrappers XPConnect generates and uses

XPConnect security membranes

Tools

xpcshell

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IRC: #developers (learn more)
Tools: JavaScript Component Wizard, Visual C++ Component Wizard, Visual C++ Component Wizard for Visual Studio 2010