A bug in Firefox is being exploited causing the browser to lock up after displaying a warning message. The message says that the machine is running a pirate Windows copy that has now been hacked.
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 also asks users to call a toll-free number within the next 5 minutes to prevent the computer "from being disabled." Folks at ArsTechnica report that the attack is currently being actively exploited on both Windows and macOS versions of Firefox.
How to get out of this Firefox Windows pirate copy bug
You can get out of it by force-closing the browser using the Windows Task Manager, or through the Force Quit... feature of macOS. BUT, if you have restore tabs feature enabled, when you open the browser again and it reloads all the previous tabs, it will end up in a loop. In that case, you can try disconnecting from the internet before opening the browser to be able to close the culprit site.
While the problem comes from scam websites, it requires no user interaction, which means even a trusted site could be used to lead users to these websites. Jérôme Segura, head of threat intelligence at security provider Malwarebytes, initially reported the bug to Firefox and has said that the company says it's working on a fix. Firefox hasn't yet officially given an update on the issue.
These scam alerts are pretty common, but, no matter the vulnerability, or bug that is being exploited, it is very important not to call or email on the displayed information to avoid further getting trapped into this mess that can eventually lead to identity theft.