The reason why IE tends to work is actually a pretty poor one.
MSIE is a lot more lenient when it comes to coding standards, as such, programmers who specifically develop on it tend to get a bit more careless with their work which leads to stuff not working right on other, higher standards, browsers. In a perfect world, all programmers would be coding to the web standards out there, but we're far from perfect. (I even do it myself). This is why things tend to work in IE that may not in other browsers.
As far as the recent issues, it's certainly something that either changed, or got fixed, with the engine that drives the Mozilla browsers. Right after FF5 was released, it stopped working for FF. It's proven to work with FF3.6 though, which really suggests either a bug in the browser, or non-compliant code on Disney's part.
Just because it works in one browser but not in the other, does not mean there isn't a bug, it just highlights how different browsers read the code differently
(Note, this post was meant to simply be informative and simply passing the time, don't read too much into it

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