What is Moonlight
Moonlight is an open source implementation of Silverlight (http://silverlight.net), primarily for Linux and other Unix/X11 based operating systems. In September of 2007, Microsoft and Novell announced a technical collaboration that includes access to Microsoft's test suites for Silverlight and the distribution of a Media Pack for Linux users that will contain licensed media codecs for video and audio.
As of Monday February 15, 2010 Moonlight 2.0 is the current stable release and Moonlight 3.0 preview 2 is the current develop release.
Uses for Moonlight
Moonlight can be used to watch online video
Watching the 2010 Whinter Olympics on nbcolympics.com
You need Moonlight 3.0 to watch the Olympics
Steps You Need to Take
If you have an older version of Moonlight installed they need to be completely removed not just disabled.
A quick way to check if you have an older version of Moonlight installed in Firefox is to go to tools > Add-ons > Plugins then look for any silverlight plug-ins If you do find any you are going to need to open Synaptic Package Manager
System > Administation > Synaptic Package Manager
Then search for Moonlight. If Moonlight-plugin-mozilla is installed, remove it. This should also remove two other packages.
After this is done, you will need to install Moonlight 3.0 Preview 2. Do this by going to http://go-mono.com/moonlight/ . On the right hand side it says "Watch the Olympics on Linux with out 3.0 preview" Click 3.0 preview, and then install the 3.0 preview 2. It should tell you to restart Firefox. (Note: When I did this, I lost some of the Firefox windows I had open)
Moonlight homepage http://www.mono-project.com/Moonlight
For more information on the development release http://www.mono-project.com/Moonlight/Preview