There may be some sort of "protection racket" (hafta system) run by some of these browsers/software. In my own surfing of the web, most of the popups ads are by these antivirus/adblockers themselves. They popup out of the blue with the message, "Is your computer at RISK? Click here." If you click, that's it!!! Your device will then be controlled by these software ganglords, who will thereafter "protect" you.
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Anyhow, one way or the other, it is good to know that we came to a happy ending in your case. Maybe your...
more... experience will also benefit other members. Thanks a lot for your diligence.