Firefox is getting MP4 support. Firefox OS and Firefox for Android on some devices has it already. Support on some versions of Windows is in nightly builds hidden behind a preference setting. Linux support is hidden behind a build switch. At some point when these backends are stable they'll be in normal builds.
Firefox getting mp4 support doesn't change anything, people are already using firefox right now, and it doesn't support mp4 right now. The fact that the marketshare of non-mp4 firefox will eventually end up small enough that most people ignore it doesn't mean mp4 is all people need right now.