Step 1 :
sudo apt-get update
sudo apt-get upgrade
sudo apt-get install libmoon moonlight-plugin-mozilla moonlight-plugin-core
Step 2 :
Go to the official Mono site to download and install the addon for Firefox.
Step 3 (Optional) :
Go to Hong Kong Jockey Club website to test the result of the installation. You may required to install Microsoft Video Media Codec. Just do so to install.
After that, restart your Firefox to make the addon works.
When the Firefox is updated to 5.0, the Moonlight is not supported. However, you can download "Add-on Compatibility Reporter" to overcome this problem.
Add-on Compatibility Reporter
That's all! See you.