Sorry this is so late a response, but I just encountered this Silverlight/ Netflix/ Firefox / Mac issue yesterday.
MacBook Pro / Mozilla Firefox / Netflix / Silverlight Update solved - DON'T DWNLOAD FROM NETFLIX
I solved this problem tonight. I have a MacBook Pro with OX 10.5.8. I know, it's old. But I love my Netflix and I recently noticed that Firefox plays Netflix much better. Then I suddenly got this message that I needed to download the latest Silverlight - it only takes 30 seconds! - WRONG. However, after much searching, I finally did the steps in order and it worked.
This was after repeatedly downloading Silverlight from the Netflix site without success. So here's what I did:
1. Went to http://www.microsoft.com/getsilverlight/Get-Started/Install/Default.aspx
2. Followed the directions. I felt like such an idiot for not doing it right before.
3. Go to your hard drive and search for "Silverlight" to locate ANY existing Silverlight files: .dmg, etc.
[also check your Libary/ Internet Plug-ins, but the above search is faster]
4. Drag it all to the trash and empty it.
5. Go back to the Get Silverlight page and click on the Install on that page, not the Netflix site.
6. Note the steps for Safari or Mozilla Firefox - I wanted Firefox, so I follwed those instructions.
7. Once it's installed, close all the browsers and Restart that bad boy. Right away.
8. Open a browser, go to Netflix and proceed to joyfully rot your brain with Netflix content. Yay!