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

Hope this is useful.

Update: [ Oct. 25, 2017 ]  -  Thought I would put the image from the msfn site that has the details on how to get H.264 (mp4) stuff working here, since I see so much of the useful and honest info on the web going dark now.  In researching a related video issue, I was surprised at how much actively wrong information was being offered, especially on Youtube.  Since folks are getting paid for generating video-views, the whole youtube ecosystem is becoming about as accurate and useful as the "sideshow" booths at the CNE that I saw as a child.  These were almost 19th-century displays of fake crap that cost a quarter, or were simple rigged games that offered low-quality payoffs even for a win.  Real carney stuff.  Youtube videos that purport to offer explanations or fixes, are often little more than sideshow-style disinfo to attract the eyeballs of the lazy.  Sad.  The internet was this great idea, that is turning into a smelly garbage-dump of fraud and disinfo before our very eyes. <Sigh..>.   I found and used this info below to get Firefox47+HTML5 H264 video stuff working right on an older Windows box, and thought I should offer the page image, in the event the msfn site goes dark - as this site here may well do soon.  Copy it, and get the Adobe stuff while you can.  I am migrating all our material over to Linux platforms (CentOS 6 and 7 series is probably best choice), but we still have a mixed bag of machines.  (A new ACER running Windows-10 is so amazingly awful and annoying, that it is comical - but that is another story for another time.)...

Detailed image-capture of info to configure Firefox to render H.264 (mp4) videos (eg. modern Youtube, as of Oct. 2017), correctly.  Test your Youtube rendering with the page "https://www.youtube.com/html5".  Once you get Firefox and the Primetime plugin from Adobe working right, you should see checkmarks in all 6 boxers on the Youtube HTML5 test screen.

Overview of What I had to do:

1) FIrst, get the plugin zip file.  I got a copy at this (backup) location, via the "wayback" machine.  The URL is:

https://web.archive.org/web/20170718145441/http://sdfox7.com/xp/sp3/EOL/primetime_gmp_win_x86_gmc_40673.zip

2) Put the zip file somewhere, and run a SHA256 check against it to confirm the hash-code that is provided on the video image.  It should start with 80975242 and end with BAB11395.  Unzip the downloaded file "primetime_gmp_win_x86_gmc_40673" and you should get three files, which are called:  eme-adobe.dll, eme-adobe.info, and eme-adobe.voucher.  These three files will have to be put into the right subdirectory in your Firefox Profile directory.

3) Install the plugin manually:  Start your Firefox, and look for where your "Profile" directory is located.  Enter "about:support" in the url box, and then, in the section called "Application Basics" click on the button beside "Profile Folder" that says: "Show Profile".  That is your profile directory.

4) In your Firefox profile directory, where the plugins are located, create a directory called "gmp-eme-adobe".  Go into that directory, and create a sub-directory called "17",and then navigate into that "17" directory you just created. 

5) Put the three files from the zip file into this new "17" subdirectory.  That's it.  You can exit the directory stuff, and in the Firefox URL box, enter: "about:config", to change the Firefox operational control parameters.  Click thru the "I'll be careful..." warning.  Make sure to be careful, eh?

6) Enter "media" in the search box, to display all the "media.<blahblahblah...>" parameters.  You will have to examine the "media.gmp...." parms shown in the screen below, and make sure your Firefox parms match these shown on that screen.  You need to BE ACCURATE.  Some parms are booleans, some are integers and some are strings.  In all cases, these terms are CASE SENSITIVE.  That means "media.gmp-yattayatta" is NOT THE SAME AS "media.gmp-yattaYatta".  Why these fine fellows mix case randomly is beyond me, but just deal with it.

7) Once you get this done (did I stress the CASE SENSITIVE thing?), you should be able to shut down Firefox, restart it, and the plugin should be installed and it should work.  You can check status of installed plugins with: "about:plugins", or just select "Tools / Add-ons / Plugins" from the top-line drop down Firefox menu.   Check status by displaying: "https://www.youtube.com/html5" and you can also check the 1-minute test videos of "Big Bucks Bunny" at "http://www.quirksmode.org/html5/tests/video.html".  (The results of the tests for my config are shown in first screen image at beginning of this section.)

Again, hope this helps.  I have spent *way* too much time on this video stuff, even building Mplayer and GMplayer from source, for my Linux boxes - and yes, got it working very nicely, in hi-res video and audio.  (I'll doc what I did in detail to get it working right at some point.)  I run the audio thru an old Denon amp that has a transformer the size of a small car-battery, and push it out thru 35 year old Ortofon 445 speakers that apparently were made in Denmark.  It all finally looks and sounds somewhat OK.  Problem is that any interesting video has to be downloaded, as youtube stops and starts with interuptions far too often.  It is sometimes unwatchable, but at least we are not using satellite, which other folks tell me is even worse in bad weather, than our poor Wimax service.

Image capture of the msfn site page that explains how to manually install the "Primetime" plugin from Adobe that lets H.264 (ie. mp4 rendering) work on Firefox-47. Installing this plugin into your Firefox plugins directly correctly on a Windows box, will make Firefox render H.264 (ie. mp4) videos. This will let Youtube work again, if you find is has been broken by the July 2017 decision to disable Youtube Flash support. Understand: Youtube is no longer offering videos using Flash. Nothing installed locally on your machine will force Youtube to offer Adobe Flash videos, despite all the disinfo on Youtube and in the useless Firefox plugins. Youtube is no longer transmitting anything using Flash. You must convert to HTML5 options. Use the Youtube test page to see if you have configured your browser correctly to render HTML5. The url for the test page is: "https://www.youtube.com/html5". Hope this helps.