No, that doesn't stop the videos from playing. It only mutes them. You still have the visual interference of the animation, along with unwillingly downloading massive amounts of video data.
That's not how it works at all... Firefox allows muted autoplay videos by default, but if you enable said flag it will prevent everything.
This is a default feature as of the latest version.
I'm using the latest version and it was set to true.
The default feature is to mute autoplay videos that have audio, so you need to enable this to prevent all autoplay.