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.

Thanks, but that didn't fix it. The Home search is set to Google. However, when you go to any page up pops Yahoo. I've searched for addons and any reference to Yahoo and I can't find it to remove it.

Start Firefox in Safe Mode {web link}

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

You may have ad/mal-ware. 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 free to use malware scanners. Each works differently. If one program misses something, another may pick it up.