The Firefox web browser since 57.0 has not had any Yahoo search so a Firefox download or update from Mozilla.org was not the cause. The Yahoo search being removed was discussed in https://support.mozilla.org/en-US/forums/contributors/712925 It could be due to a extension that was installed or updated recently so check all of your Extensions you have installed in Firefox. Some form of malware or hijack perhaps. If you are seeing this recently then perhaps something you installed or updated recently on Windows took control. https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware