I use IE8 and FireFox and cPanel 11 works fine in both of those browsers. You might want to have them clear cache/cookies on their IE and have them try again, making sure they aren't going to the SSL secure cPanel.
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Windows Firewall can prevent an FTP connection. If you are running Vista you can adjust the setting to allow FTP connections by doing the going to your Control Panel by clicking on the Windows Orb and clicking on "Control Panel". Click on "Network and Internet". Click on "Windows Firewall". Click on "Change Setting" and then click on the "Exceptions" tab. Under that tab, put a check mark next to the FTP port 43. This should allow connections to FTP sites. Click "OK" on the Firewall settings and close the other windows. I would then restart your computer and try to connect to the FTP site again. I hope this solves your problem.