You can follow the instructions on the official Moonlight website. Go here.
In short, the steps are:
1. Download the latest version (3.99 as of now) since you are using Debian Sid.
2. Iceweasel (or Epiphany) may give you a security warning. Basically, you need to allow < to install add-ons. So, change the options so that this is possible.
And you are done!