Today my wife's laptop running Windows 10 received a system message that the Windows 10 program was expiring. It asked for permission to install a new update. I allowed the update and everything seemed fine. Now, all day long while running Firefox, she keeps getting a popup yellow bar with the message "Another page is slowing down your computer". She chooses the "continue" option and within half-hour the yellow bar returns. This message continues to come up even though one tab is open!

Now the same problem is appearing on her desktop machine running firefox on Windows 10 with no update of her system. What's going on?