I am using devuan, but I think it is an upstream topic, therefore I
report it here.

There is no documentation about providing a firefox search plugin on
the start page of dwww. Maybe this/the modified files could be
added /usr/share/doc/dwww(/examples)

* sudo
cp /usr/share/firefox-esr/distribution/searchplugins/common/dwww.xml /var/www/dwww
* in dwww.xml I changed in
template="http://localhost/cgi-bin/dwww?search={searchTerms}&searchtype=d
the searchtype to =p
* in /var/www/dwww/index.html I added <link rel="search"
  type="application/opensearchdescription+xml" title="dwww"
  href="dwww.xml"> to the head section

If I surf to http://localhost/dwww/ the browser offers me to add dwww
in the search bar.