Thanks for your suggestions. It appears that as of a few days ago Safari 13 no longer supports browser extensions, rendering most ad blockers ineffective. So even though Malwarebytes' Real-Time Protection is "on," it is not blocking ads as it did previously. I opened a case twice with customer service but in a week have not received a response. Also, whenever I tried to engage Chat, even in the middle of the day, a message appeared that Chat was not available.
After researching the matter, I decided to download AdGuard on a trial basis. Supposedly this app doesn't rely on browser extensions in the same way. The pop-ups have completely stopped for now.
I would be interested to know if there are other practical, easy solutions. I suppose that there are other apps from Mac AppStore that would work as well.
Aside from the elimination of browser extensions, Safari 13 does have a number of quirks and inconveniences that still require me to use a different browser from time to time. Apple needs to fix these issues.