[Osiris]

Youtube decided to use so called overlay advertisements on selected videos which are seen as the first step to monetize Youtube. The ads seem to be ten second flash animations that appear at the bottom of the video in almost transparent form stopping the video playback until the user either clicks outside of the ad to resume the video or on the ad to be taken to the advertisers website.
The creator of Tubestop, a Firefox extension that stops autoplayback of Youtube videos, noticed that his extension did also prevent the advertisement from appearing which he attributes to the fact that Youtube ads are only displayed on the main site and not when the videos are embedded or syndicated.
Tubestop therefor stops the ads in Youtube effectively and should be downloaded by everyone who does not like to see this kind of ads on Youtube.