title: How detect a specific url in browser and get html page associate in Javascript?
body: <p>I need to modifiy a <code>html</code> page with <code>javascript</code> (fill a field).
  I have a specific url but i do not know how to listen when the browser is on the right url and get the <code>html</code> code associate.</p>
  
  <p>I do it with a webextension, so the <code>javascript</code> has to check url in the web browser and get <code>html</code> from here.</p>
  
  <p>Thank you for helping me</p>