I don't see how the web browser is the right place for codecs to live. I have multiple web browsers and multiple media players installed. Do I need six copies of every codec?
And that's part of the problem too. The Mozilla devs have made it next to impossible to plug in a video decoder to replace the embedded one, so there's no way to just punt the decision to the OS by passing the video to DirectShow / QuickTime / gstreamer / etc. It's Theora or nothing.
It seems it would be a weekend task to add a mozilla extension that replaces with embedded Microsoft Windows Media Player tags, and the necessary scripting hokum.