Outlook web access trouble

Hi,
Very strange trouble with OWA. (Exchange 2003). When acessing OWA from outside the firewall, I can read my messages correctly but when I want to write a neww message or reply to one, the body message object remain ‘disable’ and in grease color. This arrives with IE6. When using Firefox, OWA is a lighter version and no problem occurs. And when I access OWA from inside the Firewall, no problems.
Has someone an Idea ?