That is a Google ads_iframe. It goes away if I disable Enhanced Tracking Protection via the shield icon in the location/address bar. You can check for issues with Enhanced Tracking Protection. Firefox shows a purple shield instead of a gray shield at the left end of the location/address bar in case Enhanced Tracking Protection is blocking content. *click the shield icon for more detail and possibly disable the protection You can check the Web Console for relevant-looking messages like blocked content. *https://support.mozilla.org/en-US/kb/enhanced-tracking-protection-firefox-desktop *https://support.mozilla.org/en-US/kb/total-cookie-protection-and-website-breakage-faq *https://support.mozilla.org/en-US/kb/trackers-and-scripts-firefox-blocks-enhanced-track

You can also make it go away by installing [https://addons.mozilla.org/en-US/firefox/addon/ublock-origin/ uBlock Origin].