Firefox: Light Switch is a lightweight add-on that dims everything on a page but the media element that you’re interacting with, such as a video or game.
Just because the page you’re on is cluttered up doesn’t mean you have to stare at the clutter. Install Light Switch and every video and game will then have a tiny light switch next to it. Hit the switch and Light Switch dims everything on the page except for that element.
Whether you’re watching video clips or playing flash games, it’s super easy to mask the background and enjoy your media distraction free. Within the Light Switch options you can adjust the mask opacity anywhere from a faint grey overlay to a completely black mask that hides everything.
Light Switch is free and works wherever Firefox does.