Probably caused by an extension that was recently installed. http://support.mozilla.com/en-US/kb/troubleshooting+extensions+and+theme...

Had the same problem. Found it. Disable this annoying extension: "My.Freeze.com NetAssistant" (via Tools | Add Ons | Extensions | Disable ). Wish I could uninstall it entirely, but happy enough to simply cripple it so I don't get the nasty Yahoo Search! page pop-up. Good luck. Happy browsing!

I went to control panel/add-delete programs and deleted "net assistant for Mozilla". The extension disappeared and so did Yahoo.

Thank you so much! I read other forums & tried everything but this was the answer that worked for me. I've been struggling with this issue for days. What Yahoo! is doing is a reprehensible, violating thing and it is NOT okay. Best of luck to anyone else with this problem. I recommend bfox963's suggestion.

As soon as the problem started I realised that the recent update to 'Free File Viewer' that I had recently downloaded had caused the problem. The download had a default setting to install the dreaded 'Yahoo' which I failed to notice. I was able to remove the problem by doing a System Restore whilst in Safe Mode.

Yeah it was something called "freeze.com net assistant" that was causing it. Disabling the addon worked.