I'm having the same problem, i.e. the recorded tests do not run (Replay) on Firefox-6 while they run on IE8, Chrome and Safari. The Firefox browser could be launched but the execution is stuck with this line in the address bar:
I installed the Test Studio 2011.2.928.0 (Trial Version) on my Windows XP, in oder to do some Tool evaluation. I suspect this version will support FF6 since it is newer than the one you mentioned. Unfortunetely, with Test Studio Release-2 I still get this problem. I wunder if this is still an open issue or do I miss a fix?