Awesome
MozRepl 
Important Notice
Key technologies upon which MozRepl depends will be retired from the Mozilla platform in November 2017. If you are relying on MozRepl, please investigate migration paths. The last known compatible version is Firefox 54.
What is it
MozRepl is an extension for Firefox and other Mozilla applications. It listens on a TCP port for connections (by default localhost only) and responds with a shell where you can evaluate JavaScript code, in the browser context as well as in web pages.
Demo
http://www.youtube.com/watch?v=5RSnHN6S52c