My feeling is that Firefox is disallowing recursive iframes for stability reasons. In the console, I see that there's an issue with text encoding in the iframe element -- never seen that problem in my life.
The character encoding of a framed document was not declared. The document may appear different if viewed without the document framing it.
I added this:
<meta http-equiv="Content-type" content="text/html;charset=UTF-8"> to the head of the main Hype document, and now I don't get an error. But I still don't see anything. I'm stumped.