Overly broad generalisation. The vast majority of sites should not have auto-playing video or sound, however on sites where the user explicitly expects it to happen then I'd deem it as being okay. Examples are YouTube or clicking on a "Watch demo video" link - I'd regard in both cases "play video" as being implicit.
You're right. If the point of a page is to view a video, and the user has explicitly chosen to view the video, auto-play is fine. But choosing to view a video is almost like a pre-emptive pressing of "Play". Thanks for reading the post.
If someone can tell me how to disable autoplay of video in Firefox, I would be grateful.
I use NoScript add-on and Flash Block. Between the two of them, they'll stop 90%.