I stumbled upon DuckDuckGo fortuitously after installing Vimperator in Firefox around 2009 or so. If I remember correctly, Vimperator had DuckDuckGo as one of the search engine options and that's how I came to know about it.
I use Vimium with Firefox these days and it is really easy to choose either DuckDuckGo or Google for searching using the keyboard sequences od<space> or og<space>, respectively, if these are enabled as custom search engines in the Vimium options page.