Browser Disappears with gray screen, words: SafeZone Browser appear upper left

Starting today, 6/29/2012 my Firefox browser is randomly covered up completely with a gray screen with the words: SafeZone Browser appearing in the upper left.
I can’t see anything and have to close it with with green arrow at bottom right to see my browser.
Why is this happening all of a sudden? How do I make it stop? I am using Firefox version 13.0.1
cfb

http://forum.avast.com/index.php?topic=100276.msg801406#msg801406