title: Fetch HTTP-resource from an HTTPS-hosted website not working
body: <p>This issue is related only to websites hosted on HTTPS, which need to make requests to an HTTP-endpoint (seems to be working in Chrome, however).</p>
  
  <p>The following code was used:
  HTML:</p>
  
  <pre class="lang-html prettyprint-override"><code>&lt;button id="fetchButton"&gt;click me&lt;/button&gt;
  
  &lt;script&gt;
    var worker = new Worker("/worker.js");
    var button = document.getElementById("fetchButton");
  
    button.addEventListener("click", function() {
      myWorker.postMessage("");
    });
  
    worker.onmessage = response =&gt; {
      console.log("message from worker");
      console.log(response.data);
    }
  &lt;/script&gt;
  </code></pre>
  
  <p>Worker:</p>
  
  <pre class="lang-js prettyprint-override"><code>onmessage = async function (event) {
    const result = await (await fetch("http://localhost:3000/something")).json();
    this.postMessage(result);
  }
  </code></pre>
  
  <p>When hosting this website on an HTTPS-enabled website, I get back a response in Chrome, but not in Firefox, but I couldn't find any more specific docs about this one. Is there a way of getting this to work in Firefox, too? Or is this behavior (Firefox) the correct one, on how to deal with mixed-content? The above code works on both Chrome and Firefox, when hosted locally without HTTPS, so it's probably related to serving the website over HTTPS or not.</p>