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

Usually Firefox will show a built-in home page (same as the new tab page), but you might have gotten the Yahoo home page from...

• Importing that setting from another browser
• An add-on that changes your home page
• A desktop icon hacked to load Yahoo when you click it
• Intentionally or accidentally setting it yourself

Click, tools, options, home, click homepage and new window, click custom url. Paste or type a url

Now click home by the refresh button and the homepage should had changed