gstreamer's the way to go
"The idea is to start with the current stable version of the Firefox open source code - version 3.6.3 - and modify it with a new decoder. Posch tells us the project will include the open source GStreamer media framework, and this will likely handle H.264 decoding via the open GST ffmpeg codec."
This is the way to go. This sidesteps the patent problem -- open media frameworks are not patented, H.264 is. And indeed, distros that want to be strict do in fact ship with gstreamer but without H.264, DVD crypto support, MP3 support, etc. It's then up to the user to 1) Decide they are in a patent-free country or say "F" you to software patents, and get these codecs from 3rd parties (like medibuntu for ubuntu) or 2) Buy them from Fluendo -- and yes, they are available as properly licensed codecs from them that plug directly into gstreamer. My Dell Mini 9 included Fluendo codecs.
The other reason to use gstreamer, the ffmpeg implements many codecs (including vorbis and theora I think), and it's implementations seem to speed up with almost every release. So, you'll end up upgrading ffmpeg and having vlc, mplayer, and every other player *except* firefox speeding up if they throw the libs into the main executable.