I have en.wiktionary and that has an icon. In case it was a problem with Fx48 I just added the search engine for this forum, and its icon appeared within seconds.

If you use the search engine then close and restart Firefox does it appear ? (That is not a known solution, just conjecture that it may possibly help.)

No, it's not a problem that has appeared with Fx48, I've had it for a pretty long time. It used to work, but I can't remember exactly at which version it stopped working.

If I add this forum to the search engines, I get an icon, although not the favicon of the page, so I suppose that's a clue?

Here's a screenshot of what my search bar looks like. The icon to the far right is this forum that I just added, the two magnifying glasses are wiktionary and wikiquote.

To the best of my cognitive abilities, the search engines in FF seem to work with the OpenSearch protocol. What eludes me is that Wikipedia and Wiktionary/Wikiquote all have identical OpenSearch definitions, yet the icon works on Wikipedia but not on the two others.

https://en.wikipedia.org/w/opensearch_desc.php

https://en.wikiquote.org/w/opensearch_desc.php

That is a recognised alternative icon for support.mozilla.org and the icon I have on the search engine I just added. I think the icons are embeded in profile files rather than being discrete image files.

Trying in a new profile if I add en.wiktionary I get a plain magnifying glass so used instead the addon

• https://addons.mozilla.org/firefox/addon/wiktionary-en/

Try that does it work for you ?

Yes, I see the same issue (attached screenshot). I think this is related to their linking in a .ico file rather than using a data URI to encode the image directly into the XML code like the built-in seach plugins.

In https://en.wiktionary.org/w/opensearch_desc.php

<Image height="16" width="16" type="image/x-icon">https://en.wiktionary.org/static/favicon/wiktionary/en.ico</Image>

In https://en.wikiquote.org/w/opensearch_desc.php

<Image height="16" width="16" type="image/x-icon">https://en.wikiquote.org/static/favicon/wikiquote.ico</Image>

I can't tell whether failure to show linked icons was a deliberate change or a side effect of something else.