Scammers are actively exploiting a bug in Firefox that causes the browser to lock up after displaying a message warning that the computer is running a pirated version of Windows that has been hacked.
The message, which appears without any any user interaction upon visiting a site, reads:
Please stop and do not close the PC … The registry key of your computer is locked. Why did we block your computer? The Windows registry key is illegal. The Windows desktop is using pirated software. The Window desktop sends viruses over the Internet. This Windows desktop is hacked. We block this computer for your safety.
The message then advises the person to call a toll-free number in the next five minutes or face having the computer disabled.