title: Firefox extension that interacts with existing page content
body: <p>I am not a web programming expert but I would like to create a Firefox extension that rewrites pages' html and javascript code. This is a personal project so I can take my time and learn things as I go.</p>
  
  <p>I haven't been able to locate a tutorial or existing extension that does both tasks.</p>
  
  <p>Would you be able to point me in the right direction?</p>
  
  <p>Thanks you so much!</p>
  
  <p>-CxT</p>