WebM, HTML5 and the like

Hi,

in my embedding (from codebase of FF 3.6.7), when I try to view a WebM
video on YouTube (after having turned on the beta option, obviously),
the browser stops on the initial black screen with the revolving wheel
and the HTML5 logo. I tried with several WebM videos correctly working
with a regular Firefox, always getting this behavior.

Which interfaces must be implemented to get it working? I hoped it
would have worked out-of-the-box, but this does not seem the
case... :)

Thanks!
Aaron/Babele