You can remove Yahoo or at least make DDG your default search engine.

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

Firefox has not come with anything Yahoo related since Firefox 57.0 so it did not come from Mozilla or the Firefox web browser. https://support.mozilla.org/en-US/forums/contributors/712925

Maybe an extension you have installed in Firefox or updated recently has added this Yahoo search as some extensions have added such search engines in Firefox over the years.

You can check for recently installed suspicious or unknown extensions.

Start Firefox in Troubleshoot Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration or if userChrome.css/userContent.css is causing the problem.

• switch to the Default System theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• do NOT click the "Refresh Firefox" button on the Troubleshoot Mode start window

• https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems