The JavaScript Shell server for Mozilla (JSSh) is an extension for Mozilla browsers which allows remote users to execute JavaScript in the running browser.
The source code of JSSH is available at http://code.google.com/p/firewatir/source/browse/trunk/?r=131#trunk/Installation
You can download .xpi file from: https://code.google.com/p/firewatir/downloads/detail?name=jssh-WINNT-2.x.xpi
Details installation guide/instruction is here: https://testwisely.com/testwise/docs/multi-browser-support