WARNING: The moon plugin isn't fully functional yet. Not all pages with Silverlight will work correctly and some will force Firefox to die. Still looks pretty though.
Moonlight is a open source implementation of Microsoft's Silverlight runtime. It is currently in testing and shall hopefully be released later in the year. Moonlight currently still needs the mono and olive packages built from svn to create a working plugin.
Moonlight needs to be built from AUR. You can either mean download each of the packages separately or use one of the available AUR helper scripts.
The four packages needed are mono-svn, monodocer-svn, olive-svn and moon-svn. Build and install them in this order:
The install procedure for all four packages is the same, untar the tarball, then run:
from within the newly created folder.