A very handy add-on, DuckDuckGo clean. Not only does it keep your history clean, it does not include the behavioral tracking parameters included in Firefox (&t=ffcm|ffab|ffsb|ffhp|ffnt).

Here's the XML: View, Install (requires Javascript).

<?xml version="1.0" encoding="utf-8"?>
<OpenSearchDescription xmlns="http://a9.com/-/spec/opensearch/1.1/"> 
  <ShortName>DuckDuckGo Clean</ShortName> 
  <Description>Search DuckDuckGo (POST version)</Description> 
  <InputEncoding>UTF-8</InputEncoding> 
  <Image height="16" width="16">Image data removed</Image>
  <Url type="text/html" method="post" template="https://duckduckgo.com/">
    <Param name="q" value="{searchTerms}"/>
    <Param name="kd" value="-1"/>
    <Param name="kg" value="p"/>
  </Url>
  <Url type="application/x-suggestions+json" template="https://ac.duckduckgo.com/ac/">
    <Param name="q" value="{searchTerms}"/>
    <Param name="type" value="list"/>
  </Url>
</OpenSearchDescription>