This add-on scans each page for HTML 5 Video tags and replaces them with HTML 4 Embed tags. Together with an installed mediaplayer plug-in (VLC, MediaPlayer, others) this then allows one to play back any embedded video. Using the add-on preference dialogue the MIME type can be selected which is presented to the mediaplayer plug-in, as HTML 5 Video tags generally don't provide the MIME-type, and most mediaplayer plug-ins don't bother to register all the types they support. See the MIME-type list in Firefox Options for which MIME-types are registered for a particular plug-in, then pick one which is supported by your favourite plug-in.
This add-on has been tested with the VLC and Windows MediaPlayer 11 (WMP11) plug-ins. More feedback is always welcome.
There's a support channel for this add-on at irc.freenode.net