Re: A separate window popped up with a message saying "There is a new Codec Pack version. Install new version now for better performance." Is this legit?
Note that you still seem to using the old google.com/firefox site as home page.
Google stopped supporting this page quite some time ago and has hard coded a message that tell that you use an old Firefox version.
You can use the SearchReset extension to reset some preferences to the default values.