I experienced the exact same (very annoying) problem which has caused me, for the last two months, to have to log in twice to netflix to watch anything. Although I can't tell you which coding team is responsible, I feel the error came from the automatic update of the add-ons when Firefox 4 was released. My remedy was to go to the Microsoft Silverlight website and download the .dmg file again and reinstall! Macs are getting more and more like PC's all the time.....of course, we are talking about a Microsoft helper app after all!  :P Good Luck and Hope this Helps!