302 isn’t an error, it’s a redirect. Where is it redirecting you to?
Are you behind a login? There is a known issue (though it was IMPOSSIBLE to find on Google last time I looked) where Flash uses the cookies from IE, so if you’re logged onto your site in FireFox and the uploadify plugin uses Flash to make a server request, that request doesn’t have any of your FireFox cookies and it therefore not authenticated and your request gets bounced to the login screen.
I think our solution was to pass the jsessionID, cfid, and cftocken in the URL by adding them to the scriptData property of the uploadify script.
Don’t know if that your problem or not, but some HTTP sniffing should help nail it down.