Have had the same problem here since about the same time, now 12.13.10. Two help tickets to MS(N) went unanswered. Yeah, I can load it on IE but I don't use it as my usual browser and don't plan on changing just for MyMSN. Either this is a FFX problem or MS is blocking my browser.
So unless somebody steps up, buhbuhbuhbye-bye MyMSN.