user.js : user_pref("browser.search.selectedEngine", "Google");
user_pref("keyword.URL", "http://www.google.co.uk/search?hl=en&q=");
user_pref("keyword.enabled", true);
That will restore the search url back to Google.
I have same problem but i cannot find the yoog's DLL or XML that overwrite the Mozilla user preferences i need your to find it
http://www.360n.com/remove_yoog.htm << For IE not Firefox