title: Intercepting JavaScript before going to JavaScript Engine in Mozilla Firefox
body: <p>I want to develop an extension which works on scripts coming from HTTP response. I know that whole HTML code first goes to rendering engine inside browser where it is parsed to create a DOM tree. Any script embedded inside is passed to the JavaScript Engine.(Correct me if I am wrong. :) ) 
  So I wanted to intercept the JavaScript code before it is sent to the JavaScript Engine in order to modify them accordingly. </p>
  
  <p>Are there any APIs for Mozilla Firefox which would allow me to do this? How can I do it?</p>