There's a huge popup on this page that says adblock not detected and that I should install one to be safe.

Which is ironic because I'm obviously running Firefox with uBO, and they probably blocked your attempt at detect an adblocker. But if what you want to do is detect that the user is not running an adblocker, you should be testing for the positive, not the negative: your popup should only load if the code was able to do the most obnoxious, advertising-like tasks without obstruction.