Crossfire is a Firebug extension which implements a JSON protocol to allow remote clients (like an IDE or code editor) to connect to Firebug.
Who is using Crossfire ?
One of the main goals of Crossfire is to provide a simple and straight-forward method for tools in other processes to obtain the same kind of information that Firebug has about the Web pages the user is debugging.
- The Firebug Working Group is currently planning to use Crossfire in future versions of Firebug as a solution for the out-of-process plugin architecture of Firefox 4.
- The 8.0 beta version of IBM's Rational Application Developer is using Crossfire to enhance their existing Firebug integration.  Demo video
Getting started with Crossfire contains some helpful information for getting started. This is mostly intended for developers interested in building tools that connect to Crossfire, and assumes some familiarity with Firebug and Firefox extensions.
- API Documentation http://getfirebug.com/developer/api/crossfire0.1/index.html