From MozillaZine Knowledge Base

The W32/AHKHeap virus (also known as w32.USBWorm) is spread by removable drives such as USB keys. It can cause these messages to appear when starting Firefox or when visiting Orkut or YouTube:

• "USE INTERNET EXPLORER U DOPE. I DNT HATE MOZILLA BUT USE IE OR ELSE..."
• "ORKUT IS BANNED. Orkut is banned you fool, The administrators didnt write this program guess who did?? MUHAHAHA!!"
• "youtube IS BANNED. youtube is banned you fool, The administrators didnt write this program guess who did?? MUHAHAHA!!"

If your antivirus program does not find and remove it, you can use tools like HijackThis or Avast! or follow the directions posted here.