Ok I had to uninstall the mozilla-mplayer package and install gecko-mediaplayer. Much nicer!
However it still doesn't work...
I checked verbose debugging and I found TONS of these messages in .xsession-errors:
ERROR: [h264_vdpau @ 0xc3dc00]decoding to PIX_FMT_NONE is not supported.
ERROR: [h264_vdpau @ 0xc3dc00]decode_slice_header error
ERROR: [h264_vdpau @ 0xc3dc00]no frame!
ERROR: Error while decoding frame!
Not sure what order they go in...