Trying the version 29 of Fedora linux distribution I had issues to run HTML5 content in Firefox.

Some librairies are not installed by default because of license issues.

The solution is to install RPMFusion repositories:

sudo dnf install$(rpm -E %fedora).noarch.rpm$(rpm -E %fedora).noarch.rpm

and then to install the required librairies:

sudo dnf install -y ffmpeg-libs

After restarting Firefox you should be able to watch HTML5 content.