Any other ideas? Anyone? Anyone? Bueller? Bueller?
As soon as I have a minute, perhaps I'll try to trace around a bit more, but it seems pretty consistent that Shadowboxes that use iframes in Firefox are broken. If somebody else is also using mootools 1.11 and doesn't experience this problem, I'd be really interested in hearing about it. That would at least tell me that the problem is with my environment, or how I'm doing something.