The h264 video codec is the most popular codec on the internet right now. Unfortunately it's patent encumbered so it cannot be included in Firefox unless Mozilla were to pay a licensing fee. To enable h264 support in Firefox on Fedora 28 you'll need to enable the RPM Fusion repository.
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
After you've configured RPM Fusion you'll need to install some FFMpeg libraries to handle the actual h264 decoding:
dnf install ffmpeg-libs compat-ffmpeg28