Media Source Extensions Progress in Firefox

Martin Brinkmann
Mar 3, 2015
Updated • Mar 4, 2015

Users of the Firefox browser, especially those running the stable version, are still somewhat limited when it comes to HTML5 video support.

On YouTube for instance, MSE is not supported which means that some video features such as high resolutions may not be available when the HTML5 video player is used on the site.

While there is a configuration switch available that enables MSE on YouTube, it has not been enabled yet natively by Mozilla.

If you enable it, you get access to all features on YouTube that depend on Media Source Extensions such as the aforementioned high resolution playback.

According to Mozilla's Chris Double, the organization plans to enable MSE on YouTube when the stable channel hits version 37 which happens on April 7, 2015.

Linux users won't benefit from that then however as limited support will only be enabled in Firefox on Windows and Mac OS X for Mp4 videos.

Side Tip: You can enable Media Source Extensions for sites other than YouTube. Set the preference media.mediasource.youtubeonly to false to do that. There is no guarantee however that it will work on the sites you are trying it on as Mozilla is still working on adding support.

Mozilla is monitoring MSE currently in Beta, Aurora and Nightly versions of Firefox using Telemetry data. It currently only echoes the unload state and can be checked on the public Telemetry Dashboard.

There you need to select VIDEO_MSE_UNLOAD_STATE as the filter.

Two additional performance related values will be added to the telemetry data shortly. First the join latency which defines the time between video load and playback for autoplay videos, and the mean time between rebuffering.

To better understand HTML5 video playback with MSE, Mozilla created an about:media plugin that was initially hard coded into special builds of Firefox but has since then been made available as a standalone public add-on.

Interested users can download the add-on directly from Github. It is highlighting information about  MSE videos that you play in the browser.

The display is rather technical and likely only of use to Firefox or add-on developers.

One interesting bit of information from John's post about Media Source Extensions in Firefox is reference to a new preference used by MSE.

The preference media.mediasource.eviction_threshold defines the data size threshold for MSE videos. It is set to 75 Megabyte by default and data will be removed automatically when the threshold is reached to make room for new data.

The preference does not exist by default and needs to be created before it can be used. John does not mention that nor which type and value format the preference uses. From tests, it seems to use bytes to set the threshold and Integer as the data type.

If you make an error manipulating the preference, right-click it and select reset from the context menu to reset it to its default value.

Media Source Extensions Progress in Firefox
Article Name
Media Source Extensions Progress in Firefox
Find out when Media Source Extensions will become available in Firefox Stable builds to play high resolution YouTube videos using the HTML5 video player.

Tutorials & Tips

Previous Post: «
Next Post: «


  1. Chris Double said on March 4, 2015 at 12:31 am

    The eviction threshold about:config setting is type ‘integer’ and the value should be bytes.

    1. Martin Brinkmann said on March 4, 2015 at 7:52 am

      Thanks Chris. I first thought Integer but then I reset the preference and it switched to String automatically.

  2. Arun said on March 3, 2015 at 9:55 am

    Does this include VP9 support?

    1. Martin Brinkmann said on March 3, 2015 at 10:07 am

      If you enable media.mediasource.webm.enabled as well then yes.

  3. dexter86 said on March 3, 2015 at 9:08 am

    Will Firefox support 60fps videos with this?

    1. Martin Brinkmann said on March 3, 2015 at 9:10 am

      Yes that is supported then.

Leave a Reply

Check the box to consent to your data being stored in line with the guidelines set out in our privacy policy

We love comments and welcome thoughtful and civilized discussion. Rudeness and personal attacks will not be tolerated. Please stay on-topic.
Please note that your comment may not appear immediately after you post it.