The DuckDuckGo search engine is available as a Tor Hidden Service in the Tor network at:
http://3g2upl4pq6kufc4m.onion/
If you allow Javascript you can click on the “Use in Firefox” and have it immediately installed as a search plugin. However, most people surfing over Tor won't allow Javascript.
Fortunately a plain HTML version of DDG is here:
http://3g2upl4pq6kufc4m.onion/html/
but that means you can't install the search plugin with a simple click. However, click on the search bar menu arrow and you will see an extra menu option:
Add "DuckDuckGo (HTML)"
which will install a search bar plugin, but with a slightly confusing name.
If you want more control of what's happening I've made a trivial search plugin for Firefox that uses DDG's hidden service. It's a very simple file and you can check the contents yourself before installing rather than, say, trust the Javascript on DDG's own site.
To install, download:
and place it in your Firefox profile under searchplugins/. Restart
Firefox. You can then choose “DuckDuckGo Tor” in the search bar menu.
Last updated: <2014-09-11 23:26:47 CEST>