It probably changed even if you switched away from Google and back to it at some point. Firefox checks the pref value, and if it equals the default it consider

It probably changed even if you switched away from Google and back to it at some point. Firefox checks the pref value, and if it equals the default it considers it a reset and removes it from the prefs override file. There's no way to hard set the equivalent of the default value. But as mentioned elsewhere, now that the default is Yahoo, changing it to Google should stick forever.