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?
It seems it would be a weekend task to add a mozilla extension that replaces <video> with embedded Microsoft Windows Media Player <object> tags, and the necessary scripting hokum.
See: http://msdn.microsoft.com/en-us/library/dd563945(VS.85).aspx