MVC3 site using Azure ACS & ADFS continually prompts for credentials when using Firefox and Chrome

I've created a basic MVC 3 website that's using Windows Azure's Access Control Service (ACS) to perform user authentication against an Active Directory Federated Service (ADFS) endpoint. I followed the "Add STS Reference" wizard and the site runs fine and authenticates users just perfectly in IE. However, when I use Chrome or Firefox it continually prompts for my credentials over and over again.

I found this post on technet that mentions the issue as it pertains to Firefox but there's no fix for Chrome mentioned, nor do I feel like the steps required to fix Firefox are practical in the real world (i.e. I can't expect end users to do this)

http://social.technet.microsoft.com/wiki/contents/articles/ad-fs-2-0-continuously-prompted-for-credentials-when-using-firefox-3-6-3.aspx

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Has anybody else hit this snag? What am I doing wrong?

With some help from a Microsoft Rep (Adam Conkle) over on TechNet (scroll down to the comments), I finally have an answer to this problem.

Turns out, the ADFS website that performs authentication of users (this website gets setup in IIS during the installation of ADFS v2.0) is by default configured for Integrated Windows Authentication (IWA). IWA is configured in IIS to use Extended Protection for Authentication (EPA) and therein lies the problem. Apparently, most other browsers don't support EPA yet which is why Firefox & Chrome continually prompt for credentials in a loop.

Two options...

1. Keep using IWA but turn off EPA in IIS for the website (described here)
2. Turn off IWA in favor of Forms-based Authentication (described here)

I had the same issue, sort of. I had 1 person who was being prompted in Chrome and another that was not. On a hunch, I looked at the trusted sites. The person who was being challenged had the intranet site in their list of trusted sites. I removed it and tried again. This time they were not challenged.

Thought I would put that out there in case anyone else has this issue.