OS - Red Hat Enterprise Linux Server Release 6.3 (though issue also occurs with other Linux distributions such as Ubuntu)

I am looking at an issue where downloads via HTTP (using Firefox) to an external site complete with no issue on a Windows system (all versions) yet fail on all our Linux systems. Other methods of transfer (wget, ftp) also fail on the Linux systems. The download begins, then cuts off after a random period of time, usually around the 5 second mark. I believe the issue is related to IP fragmentation, as when the allowable fragmentation thresholds were raised on our firewall (Cisco ASA5585) the downloads completed. These levels cannot stay this way on the firewall due to security concerns.

I'm not a strong Linux user, as most of my work is done in Windows and we are an organization that primarily uses Windows. I was wondering if anyone could provide some insight as to why Windows would have no issue with these files, while Linux fails. Any tips/suggestions would also be welcome. I have already tried adjusting the MTU value (ifconfig eth0 mtu xxxx) on the Linux server.