title: Firefox: See what modified my HTTP headers
body: <p>How can I find out what part of my Firefox installation modifies my HTTP headers?</p>
  
  <p>Using <a href="https://martin.christianix.de/headers.php" rel="nofollow noreferrer">a tool that displays my headers for the corresponding request</a> I can see my headers contain the following string: </p>
  
  <p><code>Accept-Language: de,ar-SA;q=0.8,en-US;q=0.5,en;q=0.3</code></p>
  
  <p>I want to find out how ar-SA got in there.</p>