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: https://rpmfusion.org/Configuration:
sudo dnf install https://download1.rpmfusion.org/free/fedora/rpmfusion-free-release-$(rpm -E %fedora).noarch.rpm https://download1.rpmfusion.org/nonfree/fedora/rpmfusion-nonfree-release-$(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.