Force Internet Proxy Server

We are currently using the latest Web Marshal product for our web content manager and reporting software. We have both PC’s, Win 2K & XP, with OS X Macs on our network. Due to descisions by my superiors, some of our PC users are allowed to use Firefox web browser. I am able to lock 2K and XP users IE proxy server settings using domain group policies, but the OS X users and Firefox can, and do, remove the proxy server settings.
My Question: Does anyone know of a way to force all computers to use the web proxy server?