tell your admin he's an idiot :)
or better, don't tell him anything and install firefox without admin rights (not needed, you can install it to any directory without special rights)
Marfitalu: I didn't check it out yet, but there exists a workaround for such cases, the portable firefox. It doesn't put any files in the system drive, so you can install ist on a usb stick or so. It is all described here. Maybe that will help you (of course only, if you have a usb stick)