title: Firefox doesn't show image from cache only alt text
body: <p><strong>Observation</strong></p>
  
  <p>There is a single page application webpage served from https containing ad-slots. These ad-slots are filled by some bidding logic and the ads are written in IFRAME documents by JavaScript.</p>
  
  <p>First load shows the ad image, e.g. <a href="https://tpc.googlesyndication.com/pagead/imgad?id=CICAgKDLu47R8QEQARgBMggW4D7gy4qb5g" rel="noreferrer">https://tpc.googlesyndication.com/pagead/imgad?id=CICAgKDLu47R8QEQARgBMggW4D7gy4qb5g</a></p>
  
  <p>When user acts on the page, a new ad-bidding takes place and ad-slots are reloaded. In our case: an <code>img</code> element with identical <code>src</code> attributes is rendered.</p>
  
  <p>All other browsers (Chrome, IE, Safari) show this image, taken from local cache.</p>
  
  <p>Not so: Firefox (Windows, Mac; Version 52) acts as following:</p>
  
  <ul>
  <li>shows <code>alt</code> text on white background</li>
  <li>image is not displayed: <strong>it disappears, though it's the exactly identical image with same URL</strong></li>
  <li>when using developer tools, a tooltip on the <code>img</code> says something about "error loading image" (I exactly know only the German message)</li>
  <li>network tab doesn't show any activity on this image (also no <code>304</code> or "from cache")</li>
  </ul>
  
  <p><strong>Additional info:</strong></p>
  
  <ul>
  <li>there is no CSS active/changed which could hide the image (in fact around this img there is no CSS at all).</li>
  <li>no additional JavaScript (e.g. lazy load)</li>
  <li>no ad blocker present</li>
  <li>all resources are loaded from <code>https</code></li>
  <li>no error is logged in console</li>
  <li>first load of the page shows image</li>
  <li>no problem with all other browsers, never.</li>
  </ul>
  
  <p><strong>Example</strong></p>
  
  <p>On following page <strong>you can see the bug</strong> (I suppose it's a FireFox bug):</p>
  
  <p><a href="http://bartelt.team.netzathleten-media.de/2017-03-30_MD-230-refresh-bug/" rel="noreferrer">http://bartelt.team.netzathleten-media.de/2017-03-30_MD-230-refresh-bug/</a></p>
  
  <p>After 5 seconds the ad disappears (only in FireFox).</p>
  
  <p><strong>Questions</strong></p>
  
  <p>Can you give me some advise what could be the reason for this problem?</p>
  
  <p>Is there a way to somehow debug <code>onerror</code> of images created dynamically by foreign code (coming from ad-server)?</p>
  
  <p>This is really weird and I highly appreciate your help.</p>