Seems like something broke: http://www.owlfolio.org/media/2010/05/stegotorus-talk/ between 2013-02-18-03-11-06-mozilla-central and 2013-03-17-03-09-23-mozilla-central.
Error console shows: Error: TypeError: slides[i].setUserData is not a function Source file: http://www.owlfolio.org/media/2010/05/stegotorus-talk/r/prez.js Line: 236 See https://developer.mozilla.org/en-US/docs/Web/API/Node.setUserData The Node.getUserData and Node.setUserData methods are no longer available from Web contents. Element.dataset or WeakMap can be used instead. (See bug 842372) So, maybe this is site bug.
No other browser worked on this site, either.
Since this is a bug for multiple browsers and not Mozilla specific, I've filed an issue on webcompat.com. Closing this bug. See: https://github.com/webcompat/web-bugs/issues/39
For the record, this was never tested in anything other than the specific browser I was going to use for the talk (which was whichever version of Firefox was in beta in October of 2012) and I'm not at all surprised it has broken. (I am, however, peeved. What was so wrong with getUserData that it had to be utterly removed?) I will eventually get around to replacing my hand-rolled presentation script with something that someone other than me maintains, but it may be a while.
> What was so wrong with getUserData that it had to be utterly removed The spec was pretty insane (e.g. cloning a node subtree required calling into arbitrary user script in the middle of the clone operation, without defining what happens if that script then modifies the subtree being cloned).