<p>Today I tried it again and it worked for every site I tried (including OSM, see below). Yesterday I wasn't able to find a site where it works. So most probably this is an issue with Firefox for Android.<br>
As far as I understand they simply take the favicon, so no special preparation of the site should be needed.<br>
<a href="https://cloud.githubusercontent.com/assets/3904348/12812903/04ffae88-cb35-11e5-9e5f-7598fe69b8ed.png" target="_blank"><img src="https://cloud.githubusercontent.com/assets/3904348/12812903/04ffae88-cb35-11e5-9e5f-7598fe69b8ed.png" alt="tmp_5785-screenshot_2016-02-04-11-45-58-800170059" style="max-width:100%;"></a></p>

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