When you visit YouTube's HTML5 Video Player page -- not in beta anymore by the way -- using the Firefox web browser, you will notice that the browser does not support all of the features listed on the page.
While it supports HTMLVideoElement, H.264 and WebM VP8, Media Source Extensions, MSE & H.264 and MSE & WebM VP9 are all listed as unsupported.
If you have enabled YouTube's HTML5 player in Firefox, you may notice one limit on the site right away: some video resolutions are not offered on YouTube if MSE is not supported.
The 1080p quality is not displayed for instance if Media Source Extensions is not supported by the web browser.
While you can go around that right now by using Adobe Flash instead of the HTML5 player on YouTube, it may not always be an option considering that Flash needs to be installed on the system to make use of it.
Mozilla has been at work to change that and plans to enable Media Source Extensions in Firefox 31. You can enable Media Source Extensions right away and do not have to wait until Mozilla does so for you by following the guide below.
Note: MSE support will only be added on YouTube in Firefox 31 as it stands. While that adds support for the most popular video hosting site on the Internet, it is just the first step to full Media Source Extensions support in the Firefox browser.
To enable MSE in Firefox, do the following:
- Type about:config into the web browser's address bar and hit enter.
- Confirm that you will be careful if a warning message is displayed.
- Search for media.mediasource.enabled and double-click the name.
This sets the preference to true and enables Media Source Extensions in Firefox. YouTube will pick up on that automatically, as will other sites such as the HTML5 Test website.
On YouTube, it adds support for Media Source Extensions, and MSE & WebM VP9 but not MSE & H.264 which means that some videos may not yet using this configuration.
Update: Check this out for information on how to enable the missing parts. Note that this option may not be available yet in all versions of Firefox.
To find out if a video is supported, open it and right-click on it once it has loaded. Select stats for nerds from the context menu, and if you see Dash:yes Media Source Extensions are supported.
Media Source Extensions support is a much requested feature, especially since YouTube limits HTML5 video functionality on the site, but also since it will be used by popular video streaming services in the future to move away from plugin-based streaming.