Real has a poorly written OS-check that does not detect that Win2k3 is a superset of WinXP; the result is that the in-browser player will not load. The solution is rather easy, however. Just change the user agent:
type about:config in the address bar, and add a new string titled general.useragent.override; set it to:
Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:220.127.116.11 Gecko/20080201 Firefox/18.104.22.168
The key part here is Windows NT 5.1; this maps onto Windows XP. If you want to correctly report the version of your FireFox browser, you can get the non-overridden version by typing about: in the address bar.
The wrong way to do this is to launch Firefox in Windows XP compatibility mode. This breaks Quicktime.