title: Aria-Live in Firefox not read by screen reader
body: <p>I am having an issue where the Screen Reader is not reading the text that changes within aria-live section in FireFox.</p>
  
  <p>This is a simple example for a page where in chrome the Screen Reader reads the changes as they come in and in FireFox it does not:</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;div aria-live="assertive" id="moo"&gt;
  
  &lt;/div&gt;
  &lt;script&gt;
    let i = 0;
    setInterval(() =&gt; {
      document.getElementById('moo').innerText = 'moo' + i++
    }, 2000)
  &lt;/script&gt;</code></pre>
  </div>
  </div>
  </p>
  
  <p>Am I doing something wrong? Is there another way to announce changes when they come in besides for aria-live that people use with Firefox?</p>
  
  <p>I tested on Mac-Firefox-VoiceOver (it works on Mac-Chrome-VoiceOver)</p>