Arch Linux - Firefox H.264 Playback

When you first install Firefox on Arch it does not come with the packages necessary to playback H.264 video on YouTube (HTML5).

First off I recommend installing every possible gstreamer plugin.
sudo pacman -Ss gstreamer0.10

Next install the optional Firefox packages.
sudo pacman -S gst-libav gst-plugins-good

Restart Firefox and H.264 HTML5 playback should work.