Firefox 13: disable the most visited sites grid in the new tab page

As of Firefox 13, when open a new tab, the most visited sites appear as a grid. To disable this:

• Method 1: Click the grid icon at the top right corner
  
  
  
  
  
• Method 2:
  • Type about:config in the address field and hit enter;
  • Search browser.newtabpage.enabled and set it to false