Scammers are taking advantage of a Firefox bug that causes your browser to lock up and display the following message:
“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.”
It then tells you to call a toll-free number.
Don’t call the number!
Never ever call any number like this.
Simply force-quit your browser and go about your day.
Mozilla says it’s actively working on a fix but as of right now there isn’t one.