Firefox flex and max-width

I’m trying to display an image centered and resized by CSS (max-width / max-height, flex).

This is working fine in Chrome but not in Firefox. What’s wrong?

a {
  border: 1px solid red;
  display: flex;
  height: 100px;
  width: 100px;
  justify-content: center;
  align-items: center;
  box-sizing: border-box;
  padding: 5px;
}
img { 
  max-width: 100%;
  max-height: 100%;
  border: 1px solid green;
}

<a class="frame">
    <img src="http://ift.tt/1lBKcNH"/>
</a>

See: http://ift.tt/1Iy0qlI