In order to correctly playback HTML5 video on Fedora, you need to install the
ffmpeg-libs package, but this is not part of the core Fedora repositories. First you need to add the RPMFusion (https://rpmfusion.org) repo. RPMFusion maintains a Free and Non-Freerepo.
The easiest way to do this is to run their listed script.
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
Though you should only need the "Free" repo.
dnf install -y https://download1.rpmfusion.org/free/fedora/rpmfusion-free-release-27.noarch.rpm
Once you have the repo install siply install the package like normal.
dnf install -y ffmpeg-libs
Restart all your Firefox sessions or reboot and then HTML 5 playback should work.