This answer is quoted almost exactly from my answer to a related question: Firefox 25 and AudioContext createJavaScriptNote not a function
Firefox does support MediaElementSource if the media adheres to the Same-Origin Policy, however there is no error produced by Firefox when attempting to use media from a remote origin.
The specification is not really specific about it (pun intended), but I’ve been told that this is an intended behavior, and the issue is actually with Chrome… It’s the Blink implementations (Chrome, Opera) that need to be updated to require CORS.
MediaElementSource Node and Cross-Origin Media Resources:
From: Robert O'Callahan <[email protected]>
Date: Tue, 23 Jul 2013 16:30:00 +1200
To: "[email protected]" <[email protected]>HTML media elements can play media resources from any origin. When an
element plays a media resource from an origin different from the page’s
origin, we must prevent page script from being able to read the contents of
the media (e.g. extract video frames or audio samples). In particular we
should prevent ScriptProcessorNodes from getting access to the media’s
audio samples. We should also information about samples leaking in other
ways (e.g. timing channel attacks). Currently the Web Audio spec says
nothing about this.I think we should solve this by preventing any non-same-origin data from
entering Web Audio. That will minimize the attack surface and the impact on
Web Audio.My proposal is to make MediaElementAudioSourceNode convert data coming from
a non-same origin stream to silence.
If this proposal makes it into spec it will be nearly impossible for a developer to even realize why his MediaElementSource is not working. As it stands right now, calling createMediaElementSource() on an <audio> element in Firefox 26 actually stops the <audio> controls from working at all and throws no errors.
What dangerous things can you do with the audio/video data from a remote origin? The general idea is that without applying the Same-Origin Policy to a MediaElementSource node, some malicious javascript could access media that only the user should have access to (session, vpn, local server, network drives) and send its contents—or some representation of it—to an attacker.
The HTML5 media elements don’t have these restrictions by default. You can include remote media across all browsers by using the <audio>, <img>, or <video> elements. It’s only when you want to manipulate or extract the data from these remote resources that the Same-Origin Policy comes into play.
[It’s] for the same reason that you cannot dump image data cross-origin via
<canvas>: media may contain sensitive information and therefore allowing rogue sites to dump and re-route content is a security issue. – @nmaier