Are you sure you want that "effect"?
That Disney page is a total fail on Firefox. Blank white page.
Edit: I see it is a mobile-only page, so I guess Firefox fail is excusable.
Anyway, all they did was set the element width to 100%, so that it scales with the page. (In a supported desktop browser - which Firefox isn't for that site - the image will scale to any width.)
It's useful to learn to use Web Inspector or Firebug. They allow you to explore a site's CSS quite easily.