HTTP spec (RFC 2616) states
15.5 Content-Disposition Issues
RFC 1806 [35], from which the often implemented Content-Disposition (see section 19.5.1) header in HTTP is derived, has a number of very serious security considerations. Content-Disposition is not part of the HTTP standard, but since it is widely implemented, we are documenting its use and risks for implementors. See RFC 2183 [49] (which updates RFC 1806) for details.
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By other words the content-disposition has no meaning to IIS itself.
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I would use network sniffer (wireshark) or Fiddler to sniff client traffic and look for differences in traffic between Firefox and IE.
I'm sorry for not having better answer for you