I have the experimental Cosmic Panda lay-out on YouTube. It annoys me that videos are played automatically when I have opened a playlist.
How do I prevent this?
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Edit site preferences->
Enable plug-ins only on demand
Both Flashblock extensions block all flash content on all pages until you decide to enable it. You can also add exceptions to a whitelist to always allow automatically flash playback for certain sites.
The Opera thing will block it until you press it only on Youtube. If you use it and want to block automatic playback on all sites, go to the main Opera preferences window and select the same setting there.