While going from page to page on the site, I infrequently get a pop-up box that says
"Support Free Content"
"We use ads to keep our content free for you. Please allow ads and let sponsors fund your surfing". The pop-up blocks everything on the site.

It's gotten very annoying (once every second or third page today), and I don't even have an adblocker on.

Is this pop-up intentional?

Try blocking 3rd party cookies in browser settings then clear cookies and restart browser.