ari--e reported some research on this via IRC. Apparently it seems to be caused by Youtube's JS doing DOM introspection of a .src attribute to learn the URI scheme of the player iframe. ari--e says that if HTTPS E rewrites the iframe, the DOM .src attribute still indicates "http". Interestingly, when images are rewritten the DOM .src attirbute is HTTPS.
We need to investigate whether this is dependent on which API pathway the rewrites occurs on, and whether the #3190 (moved) patch landing in FF 20 changes the situation.
ari-_-e is working on a clean/simple reproduction case for this.