And sometimes it's IE6. No, it won't work on other browsers. No, they won't update or re-write the software - it was a nightmare to get developed the first time around.
Our software works on IE6-7 & Firefox, but yes, it can be a nightmare to support newer browsers.
By "Nightmare", I was thinking more of the dysfunctional tender + waterfall process that some large contracts (especially public or government) are prone to. If the development is so very painful, requests for change are not going to be looked kindly upon.