This is a strange situation that I'm working on to get Netflix working again for my brother.
He has a MacBook (2.4Ghz, 2GB, running 10.5.8) and is running Firefox 3.6.15 (downloaded today). When trying to view a Netflix stream, I get a message saying I must update Silverlight to the latest version. The download button gives a disk image with a installer package. After running the package, the same message is received.
Using either the add-ons window or about:plugins, Firefox reports that version 2.0.31005.0 is installed. The problem is that launching Silverlight Preferences from Applications and choosing About Silverlight shows version 4.0.60129.0, as does Cmd + I on the Silverlight.plugin file found in /Library/Internet Plugins.
Safari can view Netflix just fine, also.