Ajax features broken across browsers (on one machine)

Question

I have built a CMS system using jQuery and PHP, it has worked consistently for multiple users across different platforms over the past year or so.

Today as I was presenting it to a potential customer over the phone he ran into a problem in Internet Explorer 8, for some reason he was not able to log in to the system so I created a dirty hack to allow him in without authorizing, this allowed him to see the interface, but again, all features were still broken when it came to XMLHttpRequests, this puzzled me so I guided him through a Firefox 3.6 install over the phone with the same result, I have since checked with friends and family at remote locations and across platforms and they seem to be able to do everything he could not I have run out of ideas, so here is the question:
Has anyone ever run into this same problem and how did you solve it?

Update: I know JavaScript works, some of the other features works as intended, I have also checked for lingering console.log etc that could screw up the results, its all clear.

Answer 1

There are thousands of possible reasons, 2 that immedatly come to my mind are:

1. His/His companies Firewall might block those requests
2. He might have disabled scripts in IE8 and maybe firefox imported this setting (don't know about that)

Answer 2

Javascript and/or some of outbound requests may have been disabled completely or partially filtered by your customer's firewall/IPS/IDS. Disable JavaScript and try to reproduce the problem on your machine. If result is the same - that's your problem.

Answer 3

It turned out that the issue was with an aggressive anti-virus program and he had checked the option to "Secure my privacy", part of that "Security" was to delete new cookies.