Hmm. I am using FreeBSD desktop (FVWM) with Firefox 70.0.1 (64-bit) which in ports is version 70.0.1_2,1 and I cannot replicate any issues with the ad banners, even when I used various sized viewports. I have no ad blocker enabled for this site although I do have ublock origin installed and I can simply hide the ad container with CSS via stylus if I so choose.
So it seems that Safari and Opera both have some oddness, and macOS firefox which ought be the same as any other firefox.
Since this is definitely an issue with the default/New Style theme, I switched to it for the testing above.