FF 2.0.0.10 and IE 7 (fully patched) both gave the error on a fully patched WinXP SP2 system, so I'm guessing it's a flash problem. I'm not at work atm so I can't check the specifics on the flash being used there, but it's whatever's the latest because I had to download it to use your site. :)
I haven't run into the symptoms you described and we've been testing multiple, updated browsers (FF, IE6, IE7, Safari, Opera) on XP/SP2 and Mac. We support Flash 8 and above. Thanks for bringing this to my attention -- I'll keep an eye out.