If things go as planned, Media Source Extensions (MSE) will be available for all websites from Firefox 42 on.
The technology, used by a variety of sites and services to stream HTML5 video supports features such as adaptive streaming or time shifting live streams.
All Firefox versions pre-42 support Media Source Extensions on a whitelist only that enables the functionality on sites such as YouTube or Netflix.
That's why Youtube's HTML Video Player page lists Media Source Extensions as supported while the technology may not be available on other sites.
Firefox users could enable Media Source Extensions in Firefox since early 2014. All they had to do was open about:config in the browser and set the preference media.mediasource.enabled to true using it (it is now set to true in recent stable builds)
Additional preferences, namely media.mediasource.mp4.enabled and media.mediasource.webm.enabled are provided to add full compatibility for sites like YouTube who make use of those.
If you compare the preferences of Firefox Stable to that of Firefox Nightly, you will notice that the whitelist preference has been removed in Nightly.
Mozilla worked on MSE in Firefox ever since it was first introduced in the browser, and the organization is confident that the implementation is at a point where support can be enabled for all websites on the Internet that make use of it.
This means basically that Mozilla plans to remove the whitelist that is in use currently in Firefox 42 (currently on the Nightly channel). The effect is the same as setting media.mediasource.whitelist to false in the preferences.
Firefox users can monitor the progress made on the official bug listing 1185611 over on [email protected] There they find listed dependencies and the target milestone which, at the current point in time, is set to Firefox 42.
If you check the Firefox release schedule, you will notice that Media Source Extensions will be available for Aurora builds on August 11, 2015, for Beta builds on September 22, 2015 and for stable versions of Firefox on November 3, 2015. (via Sören)
Now You: Do you use a streaming service that requires MSE? Ran into problems using Firefox on sites requiring MSE?
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