On youtube.com/html5 it’s possible to see what
video formats are supported by the browser. For me on Fedora 25, I wasn’t able
MSE & H.264.
Start by installing RPM Fusion. When
that’s done, install the package
sudo dnf install gstreamer1-plugins-ugly
Restart Firefox and revisit youtube.com/html5. You should now have a blue checkmark on all six video formats.