I have set only one seach engine as a default - Google, however Yahoo takes over and overrides Google. This is unacceptable. I do not know how to stop Yahoo.

Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration is is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Appearance
• do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

halarm said

I cannot find Yahoo installed anywhere including under the options of search engines in Mozilla. How do I get rid of Yahoo!!!!!!!!!!!! I only want to use Google as a search engine!!!!!!!!!!

Neither the Firefox web browser nor the company Mozilla is reverting your Search Engine to Yahoo.

Something else is doing this, like say a Extension you have installed or worst case some form of malware.

Firefox from mozilla.org does not even come with Yahoo as a search engine option anymore since Fx 57 and later due in part with the legal dispute with Yahoo