title: Firefox - Intercept/Modify post data when some variable match some pattern
body: <p>I would like to know if it is possible to intercept and modify a post data when the url and some of the variables meet some pattern.</p>
  
  <p>for example:</p>
  
  <p>let the login url be: <a href="http://www.someonlineprofiles.com" rel="nofollow noreferrer">http://www.someonlineprofiles.com</a></p>
  
  <p>let the post data be:</p>
  
  <pre><code>email: "myemail@gmail.com"
  pass: "mypass"
  theme: "skyblue"
  </code></pre>
  
  <p>I would like that if:</p>
  
  <p><code>url</code> = <code>"http://www.someonlineprofiles.com/ajax/login_action_url"</code> and</p>
  
  <p><code>email</code> = <code>"myemail@gmail.com"</code></p>
  
  <p>then <code>theme</code> value be unconditionally changed to: <code>"hotdesert"</code></p>
  
  <p>Is it possible to create a Firefox add-on for that?, are add-ons powerful enough for that?</p>
  
  <p>I found this link:
  <a href="https://stackoverflow.com/questions/4791598/modify-the-post-data-of-a-request-in-firefox-extension">modify the post data of a request in firefox extension</a></p>
  
  <p>Thanks in advance!</p>
  
  <p><strong>[ADDED INFORMATION]</strong></p>
  
  <p>I don't know if it is interesting to know the version of my Firefox: <strong>35.0.1</strong></p>