Somehow, I got directed to a web site called windowssecurityhealthalert.com (DO NOT GO THERE), that opened a new instance of Firefox. It has a popup with this kind of crap in it:
WARNING!!! A security breach has been detected, your browser being hijacked and monitored. It is highly recommended to contact a Microsoft certified technician, Call toll free 1-844-798-3802 and get a scan done to resolve this serious issue.
WARNING!!! Your credentials aren’t safe, your PC is being monitored by malicious software by Trojan files installed on your PC, This is a serious security breach, This malware and Trojan agent can steal your credentials and use your PC and your TCP/IP address for criminal activity. STOP using your browser now and call a Microsoft technician at toll free 1-844-798-3802 and remove all traces of malware and Trojan files.
WARNING!!! Please stop using your browser until you get your PC scanned and removes all traces of malware, call toll free 1-844-798-3802 and get a computer scan done NOW !!!
Someone is monitoring your browser and can steal your credentials.
WARNING!!! A security breach has been detected, your browser being hijacked and monitored. It is highly recommended to contact a Microsoft certified technician, Call toll free 1-844-798-3802 and get a scan done to resolve this serious issue.
I’m running Linux.
Anyway, it won’t let me close it. Clicking on the upper-right X does nothing, and when I close the popup, it just repops. If it was Chrome I could kill the individual process for it, but Firefox only has a single process for tabs and instances. If I kill Firefox, it just comes back when I restart and restore. I cleared all cookies, but that didn’t help. Anyone know how to track down what’s allowing it to do this and killing it with extreme prejudice?
[Mid-afternoon update]
After a little research, I installed this add-on, which blocks trackers and javascript. Post installation, I was able to kill both the pop up and the instance.