Specifically, where in Firefox are you seeing Yahoo? Is your issue with Yahoo search just being in Firefox? Or are you unable to use a different search engine?

If Yahoo appears elsewhere in Firefox ... Where did you download Firefox from? Maybe you have a Yahoo special version where Yahoo is embedded in Firefox?

If that is the case, I suggest downloading Firefox directly from Mozilla. https://www.mozilla.org/en-US/firefox/all/

Then uninstall Firefox and reinstall the genuine article.

Yahoo is my search engine and is on the toolbar. I downloaded Firefox from Mozilla. If I uninstall Firefox, then I will have no way to get online. Should I install Google Chrome, then uninstall Firefox, then reinstall Firefox, and then uninstall Google Chrome?

Thanks for your help, but I hat Yahoo!

I meant hate, not hat

If your issue is that Yahoo is your search engine, just select a different search engine to be the default. No need to do anything else.

https://support.mozilla.org/en-US/kb/change-your-default-search-settings-firefox#w_set-a-default-search-engine