gistfile1.txt

js <<EOM
commands.addUserCommand(["javascriptincontent", "jsc"],
    "Run a JavaScript command in content context",
    function (args)
    {
        try
        {
            let s = new Components.utils.Sandbox(buffer.URL);
            s.window = content.window;
            s.document = content.document;
            // s.__proto__ = content.window.wrappedJSObject;
            return Components.utils.evalInSandbox(args.string, s);
        }
        catch (e)
        {
            liberator.echoerr(e);
        }
    },
    {
        completer: function (context) completion.javascript(context),
        hereDoc: true,
        literal: 0
    });
EOM