Some users have had difficulty adding duckduckgo's onion site to Firefox's default search. If you use Firefox over a Tor connection and would prefer to go to duck duck go's onion site over their typical url this extension may be for you. Simply add the extension to Firefox and click yes when Firefox asks you to change the default search engine. If you would like to see the source code please follow the link below. https://github.com/troy-ryder/duckduckgo-onion