I suspect you'll find that the code doesn't work on Safari or
Google Chrome, either. Those and AIR are all based on WebKit, so if
it fails on one, it should fail on the others.
I suspect your hand-rolled XHR code is to blame. Either get
rid of the MSIE code entirely, or, better, start using a JS
framework like Prototype or jQuery which handles these
cross-browser compatibility issues for you. The code is shorter
when you use the JS framework, which is why it's better. The fewer
LOC you write, the fewer you have to debug.