On firefox, the close button at top right doesn’t seem to show up if you have the ad-tracking blocker turned on but if you mouse over the top right corner the pointer will turn into a hand denoting the presence of a link & you can click there to close the dialog box.
right click, "Inspect Element". Add an extra CSS rule to the element{}, of display:none. This works on almost all sites with popup overlay crap.
There's probably a plugin which lets you do this with a single toolbar button...
For sites that do this, the appropriate single keypress is Ctrl-W.
Some sites appear to block Ctrl-W these days. Those sites get the full AdBlock "Go Away" treatment.