For example, in Firefox all you have to do is to right click on a search box and select "Add Keyword for this Search".

It's much better than DDG because:

1) if you care about privacy, then this is obviously the right way, as you don't have to redirect through a third-party's server

2) you can add shortcuts for whatever you want, without being limited to a fixed set. For instance I type "dex someWord" for getting the definition of words in my native language. I type "word hello" for doing a search on wordnik.com. I type "w definition" for Wikipedia. I type "mvn package" for doing a maven package search. I type "gem library" for doing a RubyGems search.

And yeah, it's easier to add a shortcut that's relevant to you, then it is to read that DDG document you linked to.