On a fresh installation of Qubes you may find that Firefox and Chromium are unable to stream certain videos, particularly those not hosted by YouTube. The Qubes documentation suggests installing VLC which is a good idea anyway but I also install ffmpeg which pulls in additional codecs as its dependencies. To install ffmpeg you will need to enable the RPM Fusion repos.
dnf install vlc
sudo bash dnf config-manager --set-enabled rpmfusion-free rpmfusion-nonfree dnf upgrade --refresh
dnf install ffmpeg