No locale of Firefox has come with Yahoo as one of the search engine defaults since Fx 57.0 as it has been removed since. https://support.mozilla.org/en-US/forums/contributors/712925 The Yahoo you may be seeing could have been added by a Extension you installed or updated recently as a few have dome so since. 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 https://support.mozilla.org/en-US/kb/search-engine-removal