I will say that when I access the windowssignin.aspx url via Firefox, I am prompted for a username/password. "Enter username and password for http://domainname.com".  My DNN account or AD account wont work.  It's almost like its looking for an admin account for AD... which I am not.

Also, I don't see any AD related errors in the DNN event viewer... just a few general exceptions that it could not load type "feature controller".. which I don't think is related to AD issue.

Yes the permissions are correct.

That you're getting the popup in Firefox shows the the permissions are correct (you need to enter your username as DOMAIN\Username). That popup is created by IIS detecting that the page requires Windows Authentication. Am I correct in assuming that the other browser you're using is IE? What version of it are you using? Also have you tried Chrome? What are the results there?

It should work with all three browsers but I'm beginning to suspect it might be a setting on your install of IE (if that is the browser you've been testing with so far).

Make sure you're using a \ between domain and username and not a /. It makes a difference (I just tested that). I think it gives you three tries and the fails to the Not Authorized page.

I don't know if Chrome uses the same internet options (from the control panel) as IE or not but it could as I couldn't find anywhere in the Chrome settings to set any options. So... all I can suggest is the usual type of things... clear your cache and cookies/possibly reset your intranet and trusted sites settings/set the browser to check for a new page every time it visits (by default it's set to automatic).