Recently any search I do via Firefox tab is automatically taken over by Yahoo. I'm not aware I ever sanctioned this and I don't want this to happen. Normally Google does the searches. Thank you

It's oftentimes a rogue extension or malware that redirects searches to something they monetize via affiliate links and similar. So you need to either disable all addons and start enabling them one by one, or restart into Troubleshoot Mode to skip loading them all for a test. See support.mozilla.org/questions/1590908 for some links and context.