[shmerl]

About gstreamer video playback: I didn't find a way to prioritize formats. Let's say some video is available in VPx (WebM) and H.264 (mp4). Firefox will pick first whatever is listed first on the page. So for me it always picks H.264 on Youtube. I prefer to use open codecs though when there is a choice, but there is no apparent way to set the priority.

[russellsprouts]

I think the browser is supposed to load them by the order listed in the HTML.

    <video>
      <source src="foo.mp4">
      <source src="foo.webm">
    </video>

Unfortunately, I think there were bugs in older browsers, so they broke if mp4 was not listed first. So most websites list mp4 first.

[shmerl]

May be an add-on can allow overriding the priority. I'll take a look if it's possible.