Here's how you can enable H.264 support for Firefox in Ubuntu 14.04.
1. Open a terminal window.
2. Type in the following commands then hit Enter after each.
sudo add-apt-repository ppa:mc3man/trusty-media
sudo apt-get update
sudo apt-get install gstreamer0.10-ffmpeg
FFmpeg plugin for GSreamer is dropped from official Ubuntu 14.04 official repositories, Firefox doesn't support the H.264 codec. So you can install it from third-party repository.More at Noobs Lab
Additional Ubuntu Resources