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From the Firefox Menu Bar/Menu Bar Tool--> Options --> Search (far left side) click GoDuckGo then remove. Alternatively leave it there just un-tick it.

Or from the far right 3 bars click and open then click Options.

Please let us know if this solved your issue or if need further assistance.

In addition to the above :

You could also type in the address bar :

about:preferences#search - and then press Enter

DuckDuckGo is a search engine.

Where do you see it  ?

DuckDuckGo Plus is an add-on : https://addons.mozilla.org/en-US/firefox/addon/duckduckgo-for-firefox/

And here's another thought : You may be dealing with DuckDuckGo the browser hijacker - malware, that you will of course want to get rid of.

Go to the 3-bar menu (or   'Tools') => Addons => Extensions and look for any unfamiliar or suspicious looking extensions, like e.g. :

SafeSearch Incognito SearchAssist Incognito Search for Fire Fox 1.6 SafeSearch ch SafeSearch net Converter.onesearch org File Convertor and Search Addon Findwide Search Engine 1.7

When you find any : remove them.

Do the same in Windows Control Panel.

However :   most malware will be 'hiding' somewhere , so let's run malware scans :

Further information can be found in this article :
https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware?cache=no

Run most or all of the listed malware scanners.
They all work differently   -   what one program doesn't pick up, the other might.

Also see : http://www.myantispyware.com/2016/10/27/how-to-manually-remove-duckduckgo-chrome-firefox-ie-edge/