Firefox search plugin using DuckDuckGo and auto-complete from Startpage.

duckduckgo.xml

<!-- DuckDuckGo HTTPS search using Startpage suggestions.
   - Url params: https://duckduckgo.com/params
   - OSX location: Firefox.app/Contents/MacOS/browser/searchplugins
   - In about:config, browser.search.defaultenginename = DuckDuckGo -->

<SearchPlugin xmlns="http://www.mozilla.org/2006/browser/search/">
  <ShortName>DuckDuckGo</ShortName>
  <Description>DuckDuckGo Search</Description>
  <InputEncoding>UTF-8</InputEncoding>
  <Image width="16" height="16">https://duckduckgo.com/favicon.ico</Image>
  <Url type="application/x-suggestions+json" method="POST" template="https://startpage.com/do/suggest">
    <Param name="lang" value="english"/>
    <Param name="output" value="json"/>
    <Param name="query" value="{searchTerms}"/>
  </Url>
  <Url type="text/html" method="POST" template="https://duckduckgo.com/">
    <Param name="kg" value="p"/>
    <Param name="kw" value="w"/>
    <Param name="ko" value="s"/>
    <Param name="ke" value="-1"/>
    <Param name="k1" value="-1"/>
    <Param name="km" value="l"/>
    <Param name="k4" value="-1"/>
    <Param name="q" value="{searchTerms}"/>
  </Url>
  <SearchForm>https://duckduckgo.com/</SearchForm>
</SearchPlugin>

Revision 1 used Wikipedia for auto-suggest terms; changed to Startpage in revision 2.