The normal behavior is to open a blank tab. An add-on will be causing it to open to Yahoo, a possible add-on doing this is the freeze net assistant. If you have that add-on you can uninstall it. The NewTabURL add-on will let you choose what you want to open on a new tab, but I would only install it if you uninstall the add-on that is opening Yahoo due to possible conflicts between the add-ons - https://addons.mozilla.org/firefox/addon/newtaburl