I had the same problem. I'm running Windows Vista and Firefox 10.0.2 browser (I think).

What worked for me was to download Safari browser, then download Microsoft Silverlight from there. 

Then I closed out Safari (I just like Firefox better) and reopened Firefox. Went to Netflix and I can watch videos again!

Hi Ryan, That worked perfectly! In my case I opened a 32 bit version of explorer, downloaded Silverlight, close explorer and reopened firefox. Working again, thank you!

Hey Steve, glad to hear it! Not having Netflix SUCKED! lol

June 30, 2012 - Here is the best solution to this problem. Uninstall Firefox, and all your personal preferences. Then go to Program File(x86) folder and delete the Mozilla Firefox folder as well.

Go the Real Player application and Preference. Then uncheck the "Enable Web Download and Recording from these installed browsers" box, and select the “Don't show (download is only assessable from right-click)”box. Close Real Player.

Do a registry cleaning with wherever registry cleaner you trust on your system. Reboot your computer.7

Reinstall Firefox. Your problems with Neflix and all streaming media are now solved. It worked for me.