this is driving me nutters.
jQuery 1.4.2, windows XP sp3
Here is my test.
Load firefox 3.5+
http://plungjan.name/test/testcors.html
w
This is a stab in the dark since I don't fully understand your problem, but I think you might be having a problem with file: URLs, which are not treated as having any origin. I'm not sure it's even possible to authorize CORS from a file URL.
file: