Aha! I found the cause of the vanishing YouTube videos. Firefox is blocking them by default now.
When I clicked on the "Info" or Lock symbols next to the page address bar, I saw that the browser is reporting that "Parts of this page are not secure (such as images)". I clicked the right-arrow to view the connection details, and then clicked "Disable protection for now". The page automatically refreshed and the videos were displayed.
Are these images truly a security risk? Is something misconfigured on the skyscraperpage.com forum itself? I don't know.