Turn Off the Lights for Chrome and Firefox is a useful browser extension that lets users obscure everything on their screen except the Flash or HTML5 video they're watching, minimizing distractions and making for a more pleasant viewing experience.

A small lamp icon is displayed in the browser menu bar or the Omnibox, and users click on the gray lamp icon to make the area surrounding the video fade. Clicking outside the video restores the rest of the screen. Users can adjust the opacity of the screen blocking and select a color other than black if desired.

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Limitations:
You're shown two introduction web pages after installation including a Welcome page. Each page has a single advertisement, and one page asks for donations.