Lately , when accessing some sites , I have been getting a blank screen with a video ad for Pandora Jewelry in the upper left corner. It takes over the tab a few seconds after opening the site. Does not seem to affect most sites (yet). Did a virus scan and a Malwarebytes scan. Both came up clean. Cookies and cache were deleted and the system rebooted. No luck. Tried restarting FF without extensions with the same results. The screen does not allow a screen shot.

Anyone else seeing this? Any clues? I am using Windows 11 Pro.