[CentOS] MP4/H.264 codec for Firefox?

Hello,

I am trying to get MP4/H.264 playback in Firefox to work on my CentOS 
laptop (for vimeo).

I installed the gstreamer plugins as described here:

https://wiki.centos.org/TipsAndTricks/MultimediaOnCentOS7

(No, I did not install Flash, VLC and all the other stuff. I only want 
HTML5 MP4 playback..)

I enabled the nux repos. I did install all available gstreamer plugins, 
i.e. -good -bad -ugly -ffmpeg etc. No success.

Has anybody got mp4 playback working?

Here's a test page:

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

Help would be appreciated!

Best regards, Roman