You may control on how location bar searches works by hidden settings.

Type in location bar "about:config", then find a setting named "keyword.enabled", by setting it "false" you disable searches from location bar.

If you just want it to revert back to Google, find setting named "keyword.url", then right click it and select "Reset".

See this article for details: Location bar search

There is a handy support article about this here:

http://support.mozilla.com/en-US/kb/Searches+are+redirected+to+another+site

For future reference, every time you install software in your computer, check all the small check boxes with adware and stuff like that.

If you have AVG software then see: AVG Antivirus and Security Software - AVG Security Toolbar Help New Tab Search

Thanks but when I do that the reset is gray so I can't. The value shows google but it still uses yahoo. Any ideas? Also I haven't updated anything since it started doing this. Thanks again.

Are you sure you are editing the keyword.URL string? There is an easy way to detect yahoo entries on about:config. Simply search for yahoo and reset everything that appears. That should do it :)