Please go to link about:preferences and search there for search engine.

You should see the default search engine option there.

Select the search engine of your choice from there.

https://support.mozilla.org/en-US/kb/change-your-default-search-settings-firefox https://support.mozilla.org/en-US/kb/add-or-remove-search-engine-firefox

Firefox is a web browser while Yahoo and Duck Duck Go are Search Engines.

Also Mozilla has not allowed Yahoo to be one of the search engines in Firefox for years now so something outside of Firefox and Mozilla is adding it, perhaps one of your Extensions installed in Firefox.