The feature is currently integrated in the nightly builds of the browser, but not enabled by default.
While the addition only affects the Windows version of Firefox, Mozilla said that the feature allows Firefox to play H.264 video, as well as AAC audio in MP4 and M4A files, and MP3 audio files without having to rely on third-party plug-ins.
To enable the backend, enter about:config in the URL bar and change the entry "media.windows-media-foundation.enabled" to "true". The nightly build can be downloaded here (opens in new tab).
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