Firefox is a web browser while Yahoo is a search engine. The desktop Firefox for Windows, macOS, Linux from Mozilla like say https://www.firefox.com/download/all/ has not come with any Yahoo search engine since Firefox 57.0 and on as you can see some discussion on that in [/forums/contributors/712925] Perhaps an extension you installed or updated recently in Firefox has added a Yahoo search engine. Or perhaps a external software perhaps did so. 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