Abak,
They didn't disclose which Firewall was in use or provide any further info. The interesting thing is that they said that the site wasn't blocked, and yet something in the firewall config was blocking access to the site.
My concern was to ensure that the problem wasn't caused by the upgrade I had just installed or by a server issue. If one client out of many is having difficulties accessing your site it really has to be something at their end causing the problem. However, thankfully it has been resolved and the client is happy, which is the #1 concern.