I was working on a project and kept getting a 401.3 error: Access denied by ACL on resource.  I would only get this error in firefox, and IE performed as expected.  Touted as a “security feature” I’m sure… Anyway, after hunting around a little and tweaking the ACL for the specific directory, I finally got it resolved.  I needed to add the Users group with default permissions to the directory.  Just having the aspnet user account and admin user didn’t resolve it, which makes sense since IIS would need it’s own permissions to access the folder.  Anyway, hopefully this bit of info helps someone in the same situation.