I try to add my local MediaWiki installation into Firefox as search engine. Firefox does not accept the opensearch response because there is an error message in the response:
$ curl http://localhost/mediawiki/opensearch_desc.php <?xml version="1.0"?><OpenSearchDescription xmlns="http://a9.com/-/spec/opensearch/1.1/" xmlns:moz="http://www.mozilla.org/2006/browser/search/"><ShortName>Trunk (de-formal)</ShortName><Description>Trunk (de-formal)</Description><Image height="16" width="16" type="image/x-icon">http://localhost/favicon.ico</Image><Url type="text/html" method="get" template="http://localhost/mediawiki/index.php?title=Spezial:Suche&search={searchTerms}" /><Url type="application/x-suggestions+json" method="get" template="http://localhost/mediawiki/api.php?action=opensearch&search={searchTerms}&namespace=0" /><Url type="application/x-suggestions+xml" method="get" template="http://localhost/mediawiki/api.php?action=opensearch&format=xml&search={searchTerms}&namespace=0" /><moz:SearchForm>http://localhost/mediawiki/index.php?title=Spezial:Suche</moz:SearchForm></OpenSearchDescription><br /> <b>Notice</b>: Uncommitted DB writes (transaction from DatabaseBase::query (DatabaseBase::query)). in <b>/srv/www/htdocs/mediawiki/includes/db/Database.php</b> on line <b>4366</b><br />