How do I remove Yahoo from my computer that is using Firefox and windows 10 ?

Where is the "tools" selection which appeared in a solution offered in this website?

Are you talking about the home page? https://support.mozilla.org/en-US/kb/how-to-set-the-home-page

Or the search engine? Type about:preferences#search<enter> in the address bar. You can select any search engine to be your default from here.

You can also remove any engine from here.

You can check for recently installed suspicious or unknown extensions.

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 or userChrome.css is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• 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

You can do a malware scan just to be sure.

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware