I assume you use an ad blocking extension? The built-in pop-up blocker watches for separate new windows. In this case, it sounds as though you are seeing either an overlay within the page or a script-triggered dialog (e.g., with an OK button). The popup blocker doesn't watch for those. The major ad blocking add-ons have forums, so you might ask on the relevant one how they suggest handling it. For example: [https://www.reddit.com/r/uBlockOrigin/]. If you have trouble finding the site, do you want to mention which ad blocker you use?