Hi, open Preferences > Search, and look at the box 'Default Search Engine.'

You can also point your mouse at the search box and should see 'Search using DDG.'

If your question is resolved by this or another answer, please take a minute to let us know. Thank you!

Hi   !
Have you done this :
3-bar menu => Options => Search and set DuckDuckGo as your default search engine under : 'Default Search Engine'  ? If you've done that :   you're using DuckDuckGo.

As for the add-on : 'Don't know what add-on it is,   but it should come with installation instructions. Maybe this will help :

https://support.mozilla.org/en-US/kb/find-and-install-add-ons-add-features-to-firefox
https://www.accessfirefox.org/Install_Addon_Manually.php

Thanks for helping an old man. It is appreciated.