If you are using Firefox for Windows and Mac, then you might be interested to know that a number of malicious sites are displaying fake warning message when you visit these sites.
“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.”
While you would be able to close the browser using Windows Task Manager or Force Close function on the MacOS, if you have settings enabled in the browser that starts from where you left, it would create an endless loop after you reopen the browser. Although you can select to disable this option later, at first you will need to close the affected site before it loads the page.
At the moment, Mozilla is working on a fix but it’s unclear when it will release a patch to address the issue.