* Support has been added for the VP9 codec in encrypted streams using Clear Key and Widevine (bug 1338064).
* Previously, MSE was only allowed to use WebM/VP8 video if the system was considered "fast enough." Now playback of VP8-encoded webm/video media is always supported, regardless of system performance.
These features could still be removed at the last minute before Firefox 54 is released on the 13th June 2017.
From the Firefox 54 for developers page:
Encrypted MediaExtensions API
* Support has been added for the VP9 codec in encrypted streams using Clear Key and Widevine (bug 1338064).
* Previously, MSE was only allowed to use WebM/VP8 video if the system was considered "fast enough." Now playback of VP8-encoded webm/video media is always supported, regardless of system performance.
These features could still be removed at the last minute before Firefox 54 is released on the 13th June 2017.