My company has a web application hosted on a client\'s machine that uses forms authentication in ASP.net. When visiting the website http://www.client.com/Application and tr
Like ajh1138 said, it's possible that IE is negotiating to "integrated auth" while FireFox is using HTTP auth or plain old form-POST.
IE may be mapping to an actual Windows username (such as the user running IE), and that user may not have access to that web page (even when the anonymous user does). That could produce the behavior you described.