H264 decoder is proprietary. Hence no default installation on fresh Fedora. We need to add rpmfusion repositories and other packages to support firefox streaming/playing h264 format.
First, install RPMFusion repo on Fedora 23:
dnf install http://download1.rpmfusion.org/free/fedora/rpmfusion-free-release-23.noarch.rpm http://download1.rpmfusion.org/nonfree/fedora/rpmfusion-nonfree-release-23.noarch.rpm
Now install the necessary h264 packages:
dnf install gstreamer1-libav gstreamer1-vaapi gstreamer1-plugins-{good,good-extras,ugly} -y
Restart your browser.