title: Firefox plugin that intercepts certain HTTP requests
body: <p>Is there a way to make a Firefox plugin intercept certain requests and respond to them without hitting the network?</p>
  
  <p>Example: every time the browser is about to request <a href="http://www.example.com/page.html" rel="noreferrer">http://www.example.com/page.html</a>, the plugin automatically returns "Hello world!" for that page.</p>
  
  <p>Cheers!</p>