Hi David

This support article should help:

https://support.mozilla.org/en-US/kb/how-to-set-the-home-page

You may also want to check to see if you have any add-ons installed that may also set the homepage to Yahoo.

You can check for recently installed suspicious or unknown extensions.

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

• switch to the Default System theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• do NOT click the "Refresh Firefox" button on the Troubleshoot Mode start window

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

No version of Firefox from mozilla.org has come with anything Yahoo related for some 5 1/2 years now /forums/contributors/712925 so I agree this may be due to an extension you have installed or updated in Firefox recently as some have done such before in past.