Hi ! First : 'Yahoo' is not a browser, but a search engine, like i.e. Google or Bing. Even though Yahoo is a legitimate search engine, you may be dealing with search.yahoo.com redirect virus, malware, that you of course need to get rid of. Would you go to Options => Add-ons => Extensions and look for anything that looks unfamiliar or suspicious ? If you find any : please remove it. Then do the same in Windows Control Panel. However : most malware will be 'hiding' somewhere , so let's run malware scans : Further information can be found in this article : https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware?cache=no Run most or all of the listed malware scanners. They all work differently - what one program doesn't pick up, the other might. Also see : https://malwaretips.com/blogs/remove-search-yahoo-com/