title: Get content of web page field to Javascript variable [Firefox Addon-SDK]
body: <p>I am using Firefox Addon SDK to develop Firefox Mozilla add-on/extension.
  Now I would like to get field value "user" from displayed web site, when the user clicks on my Add-on button.Web page's form looks like this:</p>
  
  <pre><code>&lt;form name="input" action="html_form_action.asp" method="get"&gt;
     Username: &lt;input type="text" name="user" id="user" value="name of the user"&gt;
  &lt;input type="submit" value="Submit"&gt;
  </code></pre>
  
  <p>So I need to display the value "name of the user" in my Add-on textbox, which I had created inside of the Add-on.</p>
  
  <p>But I can't figured out how to pass data FROM THE WEBSITE to the ADDON VARIABLE/SITE.</p>