yahoo browser virus

This is a browser redirect malware that affects Windows and Mac alike. You may have downloaded it inadvertently in a "free software" package or from some dubious website. To get rid of it on Mac you can use Malwarebytes which is recommended as safe by many leading users on this forum.

Also, go to Safari Preferences/Extensions, uninstall any you don't recognize, and then turn all extensions off. Test. If okay, turn the extensions on one by one until you figure out what extension is causing the problem.

 Safari/Preferences/General - see what the Home Page is set to. If something you don't recognize, see if you can replace it with something you want.

 Safari menu/Clear History.

 System Preferences and remove any Profiles you see.