Figure 3-37. Gzip compression in the Admin Panel of the Forums module.
You seem to be getting a compressed output in the forums. You are either seeing garbage, or you can see it in IE, but not in Mozilla. When you press "Refresh", Mozilla reacts like you want to
download something that is "encrypted" or compressed.
All this is a sign that you may be compressing the page twice. If, for example, your Apache has been already configured to send compressed output (with mod_gzip, or some other technique for Web Content Compression), then you should disable gzip compression in the forums administration panel (from "General Admin, Configuration", as in Figure 3-37).
See also Cannot see forum in Mozilla and see in IE.