I doubt Mozilla would want anything to do with Yahoo search engine when they even had a lawsuit with Yahoo years ago and removed Yahoo as one of the defaults as was discussed in [/forums/contributors/712925] Is this with a search in Search bar or on Homepage? perhaps an extension you installed or updated recently has added a Yahoo search. 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-suggestions-firefox https://support.mozilla.org/en-US/kb/what-search-hijacking https://support.mozilla.org/en-US/kb/how-to-set-the-home-page