This website:
contains an html5 video. You have to be opted in to html5, it says if you are on the right hand side.
This video wasn't working for me in seamonkey or firefox, until I enabled the Packman repository and installed Mplayer. So I think there are some dependencies which are not satisfied by installing firefox. But I think I read that firefox downloads some things on the fly for html5, so maybe it should check for libraries too?