title: Firefox content script not loading in some pages
body: <p><strong>Context</strong></p>
  
  <p>I am currently working on a browser extension which is working as expected with Chrome and Opera but I am facing issues with Firefox. Here is a minimal version of <code>manifest.json</code> needed to reproduce the problem:</p>
  
  <pre><code>{
      "name": "Example",
      "version": "0.0.1",
      "author": "Pyves",
      "content_scripts": [
          {
              "all_frames": true,
              "matches": [
                  "&lt;all_urls&gt;"
              ],
              "js": [
                  "content.js"
              ]
          }
      ],
      "manifest_version": 2
  }
  </code></pre>
  
  <p>And here is the related <code>content.js</code>:</p>
  
  <pre><code>console.log("Content script loaded");
  </code></pre>
  
  <p><strong>Issue</strong></p>
  
  <p><code>Content script loaded</code> is systematically logged regardless of the visited page when using Chrome and Opera. Nevertheless, the content script doesn't seem to load in some pages when using Firefox, for instance raw GitHub pages such as the following:
  <a href="https://raw.githubusercontent.com/badges/shields/master/README.md" rel="nofollow noreferrer">https://raw.githubusercontent.com/badges/shields/master/README.md</a></p>
  
  <p>There are no error messages in the Firefox console stating why the content script was not executed on that particular page.</p>
  
  <p><strong>Questions</strong></p>
  
  <ul>
  <li><p>Why is the Firefox extension unable to load the content script into some pages?</p></li>
  <li><p>What changes need to be made so that the extension works consistently on all browsers?</p></li>
  </ul>