Firefox currently autoplays video that has no audio. This is a delicate balance. If browsers disable autoplay by default, then advertisers will find other ways to push video ads in users' faces. I read an anecdote that Google tried disabling autoplay on mobile and advertisers started playing video ads using an H.264 video decoder in JavaScript that paints video frames to a canvas. So users still see the video ad, but now using more bandwidth, memory, and CPU.