reference:
http://www.croczilla.com/bits_and_pieces/jssh/
JSSh - a TCP/IP JavaScript Shell Server for Mozilla
JSSh is a Mozilla C++ extension module that allows other programs (such as telnet) to establish JavaScript shell connections to a running Mozilla process via TCP/IP. This functionality is useful for interactive debugging/development of Mozilla applications, remotely controlling Mozilla, or for automated testing purposes.
Note that JSSh is a binary component, which makes it a bit awkward to distribute as a firefox extension. There is a also a javascript-only component that apparently pretty much does what JSSh does. For details see Shane Caraveo's blog post here: jssh replacement? SD Connector. I haven't tried it out yet, but it looks promising!