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.
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.
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