I usually have screen set to blank after 20 minutes and use Presentation Mode for watching longer videos in Firefox.
VLC and smplayer on full-screen can disable screen blanking or screensavers but Firefox cannot
It is more a like workaround than a “proper” solution from Firefox or Xfce maintainers
Issue with screen blanking seems to be affecting older installs that are probably still using light-locker and do not have xfce4-screensaver installed