Firefox Untitled Tab When I Open A New Tab (Untitled) Shows Instead Of My Most Visted Websites. How Can I Revert It Back?

When I open a new tab (Untitled) shows instead of my most visted websites. How can I revert it back? - firefox untitled tab

I have the latest Firefox and the latest toolbar from Google. I am running Windows Vista Home Basic. If you help me, please help as soon as possible.