Did you read these instructions? https://support.mozilla.org/en-US/kb/add-or-remove-search-engine-firefox

Your Add-ons list in the troubleshooting information doesn't show a Yahoo search engine. This means that you either have an extension that installs this or you have malware. You can do a malware scan just to be sure. *https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware Start Firefox in <b>Troubleshoot Mode</b> to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if <b>hardware acceleration</b> or if <b>userChrome.css/userContent.css</b> 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 ----- If clearing the "Restore Default Search Engines" on the "Settings -> Search" page doesn't work, try to rename/remove search.json.mozlz4 in the Firefox profile folder with Firefox closed to rebuild the search engines registry. You can use the button on the "Help -> More Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the <b>about:profiles</b> page (Root directory). *Help -> More Troubleshooting Information -> Profile Folder/Directory:<br>Windows: Open Folder; Linux: Open Directory; Mac: Show in Finder *https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data Firefox will rebuild search.json.mozlz4 from the built-in search engines.