Things have changed a bit lately with firefox, gstreamer1.14, ffmpeg, rpmfusion....
To get all firefox quantum video to run under fedora 28 you can't just add ffmpeg-libs - You also have to add compat-ffmpeg28 from rpmfusion....
Code:
dnf install https://download1.rpmfusion.org/{free/fedora/rpmfusion-free,nonfree/fedora/rpmfusion-nonfree}-release-$(rpm -E %fedora).noarch.rpm
dnf install ffmpeg-libs compat-ffmpeg28
to check if everything(including h264) will work now in youtube: https://www.youtube.com/html5
note: I am not sure you even need ffmpeg-libs anymore?
why you are here in rpmfusion - don't forget to add your gstreamer1 video plugins for your fedora programs....
Code:
dnf install gstreamer1-libav gstreamer1-plugins-ugly
or you could just do everything at once listed above(need unrar?)....
Code:
dnf install https://download1.rpmfusion.org/{free/fedora/rpmfusion-free,nonfree/fedora/rpmfusion-nonfree}-release-$(rpm -E %fedora).noarch.rpm
dnf install gstreamer1-libav gstreamer1-plugins-ugly unrar compat-ffmpeg28 ffmpeg-libs