Yahoo in this case is a search engine while Firefox is a web browser and not a search engine. Mozilla the makers of the Firefox web browsers does not have a search engine of their own. https://support.mozilla.org/en-US/kb/add-or-remove-search-engine-firefox https://support.mozilla.org/en-US/kb/add-custom-search-engine-firefox https://support.mozilla.org/en-US/kb/change-your-default-search-settings-firefox In case it is about making Firefox teh default web browser on Windows then see https://support.mozilla.org/en-US/kb/how-change-your-default-browser-firefox-windows