title: Firefox addon shows name but no icon in Customize window
body: <p>I have recently developed a Firefox addon using the SDK.
  However, once installed the icon does not show, but only the name of the addon in the Customize window for the addon bar (or the menu in Australis).
  I haven't seen this behavior in other addons, so I'm guessing it is probably something I did wrong.
  Maybe I didn't include the right sized icon? My package.json contains the lines:</p>
  
  <pre><code> ...
   "icon": "data/icons/icon32.png",
   "icon64": "data/icons/icon64.png",
  ...
  </code></pre>
  
  <p>The icons display well in the widget and in the addon page.
  Shouldn't that be enough?</p>