FreeBSD OpenSearch plugins (not working)

[Gerald Pfeifer]

The OpenSerach plugins offered
  http://www.freebsd.org/search/opensearch/
appear very useful and I know I've used one or the other in the past.

Alas, when I select one of these 
(http://www.freebsd.org/search/opensearch/portoverview.xml, say)
in Firefox 23 but also older versions, all I get so see is the
raw XML ("This XML file does not appear to have any style information 
associated with it. The document tree is shown below.") and there 
does not seem any natural way to actually add the plugin.

How can I add one of those?

Does it work for others?

Gerald