Use with XULRunner - bard/mozrepl GitHub Wiki

Simply put MozRepl XPI package under your application’s extensions/ directory (ensuring that the expanded xpi’s directory or the .xpi filename matches its em:id in install.rdf) and launch the application like this:

    $ xulrunner application.ini -repl

You can optionally specify a port number after the -repl switch, otherwise the default 4242 will be used.

If you get tired of typing -repl on the command line, you can also set the extensions.mozrepl.autoStart preference to true.

Note: This technique also works for Thunderbird, and (probably) any other Mozilla-based product that supports extensions.