That is coming from the site not from the Browser. Depending on what popup Addon your using might be time to find one that works. Firefox has basic blocker or popup blocker anything more will be up to the user to decide what Adblocker Addon they find and use.

Haven't seen a pop-up in I don't know how many years; 15 maybe. Just curious how this one is managing to function, suddenly.

Dropa was right, it is coming from the site. see screenshot Just a newsletter sign-up and not really an ad. It looks like Adblock Plus can block it. see screenshot

ABP blocked and I clicked around for a few minutes and never saw it again.