About the on the same machine problem with firefox, what do you get on firefox?
Can you control proxy configurations (and relative exceptions) on both browser to see if and how they differ?


About the on a different machine problem, since you get a 500 ("internal server error") error you reach at least a server. Is there a proxy between the client machine and the IIS server? Can you check IIS log to see if requests from the client machine reaches the server?
Is the error an ASP error with a stack trace?