Firefox ver. 47 "about:config" media parms, and test-cases from www.youtube.com/html5 and www.quirksmode.org/html5/tests/video.html, showing Firefox ver. 47 working correctly on a WinXP/SP3 box - full HTML5 support. My site here (from simplesite.com folks in Denmark) can nicely serve videos, but on my Firefox platform, I couldn't see them, until I fiddled the about:config parms, and installed the Primetime Content Decryption Module from Adobe. Even if there is no DRM present, you can use PCDM stuff to enable MP4 (H.264+ACC) in your older (stable) Firefox. See how the Youtube HTML5 test page now shows checkmarks in all boxes, and the quirksmode.org HTML5 test pages (with "Big Buck Bunny"), renders all three video examples. Details on how to get FIrefox to work right under older WIndow platforms is from the MSFN.org site: http://www.msfn.org/board/topic/175591-enable-mp4-h264-aac-html5-video-in-firefox-on-windows-xp-without-flash/?_fromLogout=1
Hope this is useful. I want to be able to serve up videos without the dependency on Youtube or other restricted US-based sites.

Firefox and HTML5 .mp4 Video rendering.

Getting Firefox 47 working properly with Video:

I run a version of Firefox that does not have .mp4 video rendering, and to make it work on my older WinXP/SP3 box, with both Flash and HTML5, I followed the instructions on this site, and they worked well.  As the test pages montage above show, I can render all common/popular video formats now.

http://www.msfn.org/board/topic/175591-enable-mp4-h264-aac-html5-video-in-firefox-on-windows-xp-without-flash/?_fromLogout=1

Hope this is useful.  I'm experimenting with verison 55.0.2 version of Firefox, but as the Mozilla folks have decided to drop support for all plugins, and are also taking a heavy-handed approach (again) at logging all one's web activity (the "Top Sites" stuff is back, and cannot be disabled in newer FF versions, apparently), I am remaining with this older version for now. 

Also, experiments on Android Firefox (which can have the Adobe Flash drivers successfully installed and made operational), show that Android 6.0.1 (current version for Samsung Tab-A) and Firefox 55.x version, are much less reliable than older Kitkat (Android version 4.4.2, running on Samsung Tab-3, with Firefox 40.x).  The Adobe Flash driver (version 11.1.115.81) can be installed on Android devices, as an .apk file, by following the instructions provided on the site below:

https://helpx.adobe.com/flash-player/kb/installing-flash-player-android-devices.html

I can confirm that Firefox version 55.0.2 (most current production Firefox, as of Sept. 14, 2017 from Google Play Store) with Android 6.0.1 on Samsung Tab-A (most current Andriod that Samsung official makes available for the device) works with the Android Flash drivers version 11.1.115.81, which are indicated on the Adobe documentation as only working on Android 4.0 (I don't know what sugar candy version this is ... Ice Cream Toad?).

The Flash Video archives from Adobe are at the url below.  You can find the Android Flash Ver 11.1.115.81 about 3/4 ers of the way down the page.  To download and install, make sure you fiddle the "Settings" in Android so that you can install .apk files (Android Package Files) from a non-certified source. 

https://helpx.adobe.com/flash-player/kb/archived-flash-player-versions.html

Really hope this is useful.