Type about:addons<enter> in the address bar to open your Add-ons Manager. Hot key; (<Control> [Mac=<Command>] <Shift> A)

In the Add-ons Manager, on the left, select Extensions. Remove “ yahoo “ Anything.

Also remove SafeSearch, SearchAssist Incognito, FileConverter Addon, SearchLock, DiscreteSearch, WebSearch, Findwide Search Engine

Now do the same in the computer’s Programs Folder.

Windows: Start > Control Panel > Uninstall Programs. Mac: Open the Applications folder. Linux: Check your user manual.

Actually as of Firefox 57.0 and later any locales of Firefox that had Yahoo as one of the default search engine options was changed back to Google. As of Firefox 59.0 and 52.7.0esr and later Releases the Yahoo search engine was completely removed and not just hidden. https://blog.mozilla.org/blog/2017/11/14/firefox-features-google-as-default-search-provider-in-the-u-s-canada-hong-kong-and-taiwan/ https://hg.mozilla.org/releases/mozilla-release/rev/a46d7b315f70

Little public information on the real reason(s) for the complete removal however Mozilla and Yahoo have been in a legal dispute for some months now. https://blog.mozilla.org/blog/2017/12/05/mozilla-files-cross-complaint-against-yahoo-holdings-and-oath/

So Firefox downloads since 59.0 and later from mozilla.org or www.mozilla.org/firefox/all/ and updates does not even come with Yahoo as a search engine option on any locale anymore.

The getting changed to Yahoo or what appears to be a Yahoo search engine is due to some other means outside of Firefox adding the search engine or due to malware or due to an extension you installed.

For a time there were some people who got a malware called Yaahoo for example.