H.264 videos that played in Firefox 34 won't play in Firefox 35.0

Question

We host a ton of h.264 videos in the mp4 container displayed through JW Player. We had zero problems with Firefox 34 and earlier playing these videos, the video is H.264 and the audio is AAC, the whole thing is around 500 kbit/s.
They play fine on Firefox 34, Chrome and IE but after updating to Firefox 35.0 we get an error: "Error loading media: File could not be played".
We have found a fix to get them to work in the about:config, switching "media.fragmented-mp4.enabled" to false OR switching "media.fragmented-mp4.exposed" to false will allow our videos to play. Single file mp4's will also begin to download where before we would get a "Video can't be played because the file is corrupt."
Alternatively for Mac users if they just set media.apple.mp4.enabled to False it also works fine.
Non h.264 mp4 videos will play fine.
Here is a playlist that won't play: http://media.vineyardinstitute.org/other/vi_announcement/firefox-test.html
Here is the single video file: http://media.vineyardinstitute.org/other/vi_announcement/05_kingdom_theology.mp4
Is there anything I can change on either the playlist or the video files to get this to work? Why would the h.264 files that worked in Firefox 34 NOT work in 35? I will not be able to re-encode all of the videos, it would take me months. Is this a bug with Firefox?
Also I have tried this in safe mode, tried with every plugin deactivated, I have uninstalled Flash and the videos would not play as well as reinstalling a new version of Flash and a fresh copy of Firefox 35.0
Any help would be much appreciated, we are currently directing all of our users away from Firefox till we get this resolved.
Thanks,
Kyle

Answer 1

Do you have H.264 codec enabled in the plugins list of Firefox?
Does this happen in the most recent version of JWplayer?
There was an error loading the media:
"Media resource http://media.vineyardinstitute.org/other/vi_announcement/05_kingdom_theology.mp4 could not be decoded. firefox-test.html
13:43:48.280 "Error playing media: " MediaError { code: 3 } jwplayer.js:11:373
13:43:48.280 "CAPTIONS([object Object])""
jwplayer("myElement").setup({
playlist: [{
image: "http://media.vineyardinstitute.org/other/vi_announcement/images/05_kingdom_theology.jpg",
sources: [
{ file: "http://media.vineyardinstitute.org/other/vi_announcement/05_kingdom_theology.mp4" }
title: "Kingdom Theology",
description: "A look at what makes VI theologically unique.",
height: 720,
listbar: {
position: 'bottom',
size: 240,
thumbs: 'false',
repeat: 'list',
width: 640,
You might also find better help with this error on the JWplayer support site:
*similar topic/error: [http://support.jwplayer.com/customer/portal/questions/6062663--error-loading-media-file-could-not-be-played-error-in-jw-player-6-while-playing-mp4-with-chrome-html5]
In that one there were two errors: needed a higher version of Flash installed and the codec issue. As you mentioned this had been narrowed down to the codec of the mp4 file. And this codec is added with the default cisco h.264 player. [https://gigaom.com/2014/10/14/h-264-support-arrives-in-firefox-thanks-to-cisco-but-h-264-web-videos-still-wont-play/]