Seriously - there is not decision been taken here. This is just a discussion about gecko on Android, and the implication for Desktop.
Saying "Firefox will support H264 and MP3 decoding with OS codecs" is just speculation.
Edit: to be clear: I am saying that there's no plan to support H264 on Firefox Desktop.
Quoting directly from the link:
Initially this will be enabled on Gonk (B2G). In a few weeks we will add support for Android as well. We will support encoding any video/audio format that is supported by existing decoders present on the system, including H.264 and MP3.
Of course it's possible that the bug doesn't land. But calling this speculation? Please.
I do not see what benefits it could have.