The Web Slingplayer plug-in for the PC version of Firefox and Intenet Explorer has a hidden key combination of pressing the [Ctrl] while clicking on the streaming window which will give you a statistics window.. I don't know if the Mac version has something similar or what the hidden keystrokes could be, however, if you find it then you can see exactly what's happening.
I have the same, exact issue on my Mac Mini. In Safari the highest quality is "Best" while in Firefox it is "Best/HD." There does appear to be a slight quality difference as the Safari stream looks more pixelated. The interesting thing is that on my Macbook Pro running the same version of Safari (5.0.1), I still get "Best/HD." I have no idea why. It only started happening a few weeks ago. Any help would be appreciated.