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Well, Google's stopped playing video ads on YouTube for the time being, meaning you'll be hard pressed to have any fun removing the ad for yourself.
A Firefox extension called TubeStop was created to keep YouTube videos from automatically playing on the video-sharing site. A great side effect of TubeStop has been the removal of the new ads on YouTube clips. How does this work? TubeStop replaces YouTube's native player on the site with the embeddable version, which has not been ad-ified yet. So get the TubeStop extension now, and you'll be prepared for when Google starts rolling ads on YouTube again.