The FFmpeg plugin for GStreamer 0.10 is not available in the official Ubuntu 14.04 repositories and because of this, Firefox doesn't support the H.264 codec.
Furthermore, without the FFmpeg plugin for GStreamer 0.10 package, KDE apps can't play H.264 videos in Kubuntu 14.04 if they use the GStreamer backend. Another issue with this is that Amarok can't play WMA files. There might also be other affected applications.
The bug was reported on Launchpad and it's not yet known if it will be fixed. Until (if) it's fixed, here's a work-around you can use to get Firefox and KDE apps that use the GStreamer backend to support H.264 in Ubuntu 14.04: basically, all you have to do is install gstreamer0.10-ffmpeg by using a PPA:
sudo add-apt-repository ppa:mc3man/trusty-media sudo apt-get update sudo apt-get install gstreamer0.10-ffmpeg
Here's the YouTube HTML5 page under Firefox in Ubuntu 14.04, before and after installing gstreamer0.10-ffmpeg: