Firefox could not install the search engine from…

I'm trying to make APLcart work with OpenSearch, but keep getting Firefox could not install the search engine from: https://aplcart.info/opensearch.xml with:

<link rel="search" type="application/opensearchdescription+xml" title="APLcart" 
href="/opensearch.xml">

Where /opensearch.xml is:

<OpenSearchDescription>
<ShortName>APLcart</ShortName>
<Description>
Search APLcart: A novel approach to finding your way in APL
</Description>
<InputEncoding>UTF-8</InputEncoding>
<Image width="16" height="16" type="image/x-icon">https://aplcart.info/favicon.ico</Image>
<Url type="text/html" template="https://aplcart.info/?q={searchTerms}"/>
</OpenSearchDescription>

• Note that this is not the same issue, since I do have Url type="text/html".
• I've tried with method="get"

What do I need to change for my OpenSearch specification to be compliant?

You have to use the correct namespace

<OpenSearchDescription xmlns="http://a9.com/-/spec/opensearch/1.1/">

If you open the browser console (CTRL+Shift+J) you can see the error message

Invalid search plugin due to namespace not matching.