The message is inaccurate. It should more accurately say that
your browser needs to be able to play H.264 HTML5 video.
TenFourFox is perfectly capable of playing HTML5 video, but it
only supports the WebM codec right now. H.264 support has
complications of patent protection and proprietary codecs, and
while I would like to add this support, the work to do so is
substantial. As a result, it will not occur in either the
TenFourFox 31 or 38 timeframes. If you need to watch this video,
unfortunately you will need to use another browser.