Thank you for your personal message. The cowboybebop link that you sent to me is tripping my Malwarebytes Anti-Malware Premium, which has real-time protection. It is reporting blocking a malicious site each time I go there. That site is probably infected, and the malicious redirection may be coming from that site.
I know that you have Malwarebytes Anti-Malware (MBAM) Free on your computer. I would like to have you try a test for me, if you would be so kind. Please go here and follow the steps to do a clean uninstall of MBAM Free. Then I want you to install a new version of MBAM, but this time, select the 30 day free trial of Premium (if you don't want to buy it, it will automatically revert to the Free version after 30 days). Please go to that bebop site and see if you get a similar warning.
More importantly, when you exit your browser, do you still see the pop-up? I am guessing, among some other possibilities, that the MBAM "blocked" site is opening a new window, behind the scenes, that you see when you exit your browser. That would be the simplest explanation and the one easiest to deal with.
I use both MBAM Premium and Malwarebytes Anti-Exploit Premium, to complement my Bitdefender Total Security. There are other good products as well out there (Emsisoft, Kaspersky, etc.,) so this is not a Bleeping Computer endorsement of my security configuration. I am just sharing with you what I use. The great thing about MBAM Premium is the real-time protection. It prevents malware that it can identify from being installed, unlike the Free version that detects it only when the user runs a scan after the malware has been installed. It is easier to prevent infections than to clean them up afterwards.
If that does not resolve your pop-up issue, then I will refer you to the Virus, Trojan, Spyware and Malware Removal Logs Forum. There we can run more sophisticated scan and fix tools to try and locate the critter and eradicate it. Don't be concerned because I will stay with you. I am at the point in my training where I can work on those logs in that Forum, under supervision. So, if we need to go there, I will make arrangements to pick up your logs and also snag the supervisor who I have already consulted about your situation. You are in good hands. We will do our best here at Bleeping Computer to rid you of that pesky pop-up.
Please let me know how you make out with the MBAM Premium. I am hoping that it solves your problem the simple way.
Have a great day.