Sounds to me like mozilla is actually trying to protect the information of the browser user, and IE isn't. First of all, why would you need to know what path the uploaded file came from? Secondly, why should a user want to give you that information?
(EDIT)
Some light reading:
RFC 2183 , specifically section 2.3 "The Filename Parameter"
Specifically:
The receiving MUA SHOULD NOT respect any directory path information
that may seem to be present in the filename parameter. The filename
should be treated as a terminal component only.
Also, the section "Security Considerations"