Troubleshoot Firewall Issues
If the user can't see the login page, can they access the WiDirect through its IP address? The URL http://10.4.1.1 is just a blank page, but http://10.4.1.1/portal/admin would bring you to the admin login page. This would verify that they are properly connected.
Next, can they see the "authentication" part of the WiDirect? This is a web server that runs on a different port than the standard web server. You can check that it is accessible by going to http://10.4.1.1:8060/, which should bring you to the login page. If they can't access that page, then something on their computer is blocking them from that port (a firewall). It is possible that a firewall or internet security program is allowing a different level of access for Firefox and Internet Explorer.
Perhaps their browser it is set to use a proxy server. Verify that their proxy server setting is turned off
Any firewall programs, such as from AVG, Norton, Symantec, or ZoneAlarm, can be temporarily disabled to see if they are what is causing the problem. If it works after they are disabled, then they should be reconfigured when enabled again.
If these problems persist, it would be helpful to know what versions they are running of Windows, Firefox, and their Internet security software.