Why Firefox 38.0.5?? Since Nightly is more like Firefox 45, there are bound to be differences as media features are tweaked.

Did you install the full media feature pack?

Download Media Feature Pack for N and KN versions of Windows 10 from Official Microsoft Download Center

My FF must have been bugged because it showed as "up-to-date". Anyway, manually uninstalled it, downloaded newest version, still same problem even with clean profile.

I might add that Nightly is not working for me anymore. I did recall it working at one point but now it stopped it seems.

YouTube HTML5

So do you have the full media feature pack installed? Firefox needs it to decode MP4 video through Windows Media Foundation.

If that is installed but YouTube won't serve you MP4, on the 360p WebM video, could you check that the following is not disabled:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful.

(2) In the search box above the list, type or paste media. and pause while the list is filtered

(3) If the media.mediasource.mp4.enabled preference is bolded and "user set" to false, double-click it to restore the default value of true

Yes, I installed the same package you linked.

media.mediasource.mp4.enabled;true Was true by default.

If you return to about:config and scan down the list of media. preferences, is anything customized in a way that looks like it might disable anything? Particularly anything with mp4 in the name?

There is a big list of Excluded features on this page that include the codecs:

• https://support.microsoft.com/en-us/kb/3010081

MPEG, WMA, AAC, FLAC, ALAC, AMR, and Dolby Digital audio codecs: Enables the playback of digital audio content such as MP3, WMA, MPEG-2 audio, AAC audio, FLAC and ALAC audio, and AC-3 audio.

VC-1, MPEG-4, H.264, H.265, and H.263 codecs: "Standards-based codec components" (as they are collectively called) enable Windows 10 program such as Windows Media Player to support activities that include the playback and creation of multimedia files that are encoded with standards-based codecs. 

jscher2000 said

If you return to about:config and scan down the list of media. preferences, is anything customized in a way that looks like it might disable anything? Particularly anything with mp4 in the name?

Nope. But I found out, that Groove-Music is also complaining about the Media Feature Pack not being installed. Even though when I try to install the Media Feature Pack from your link it says, it is already installed. Problem is, it is an update which cannot be simply uninstalled.

So I rolled back to a System Restore Point where the Media Pack was not installed and re-installed it. Still same problem with Groove and Firefox. This seems to be a Windows 10 N issue.

I think Google's detection page (https://www.youtube.com/html5 -- your earlier screen shot) is pretty good evidence that your Firefox is not properly detecting or connecting with the Media Foundation components. But if they are installed, and the preferences are not disabled, then I don't know why Firefox isn't using them.

If you start Windows Media Player locally and point it to an MP4 file, can it play it?

That's the curious thing: Even with the Media Feature Pack installed, there is no Windows Media Player. This has to be some kind of Windows bug.

If you save an MP4 to the desktop and double-click it, what program plays it? Perhaps there's a new name for Windows Media Player in Windows 10?

You can right-click > Save Link As on this link:

http://www.quirksmode.org/html5/videos/big_buck_bunny.mp4

It's from this page:

http://www.quirksmode.org/html5/tests/video.html

Thanks, already checked but there is no Windows Media Player.

I posted in the Microsoft Forums as this seems to be a Windows issue. http://answers.microsoft.com/en-us/windows/forum/windows_10-other_settings/windows-10-n-media-feature-pack-not-working-after/953d600b-badd-4fdb-8756-6eb3b740d02e