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3D stereoscopic videos do not work with NVIDIA's 3D Vision technology and 3D displays in Firefox 11. Instead, you may see a 2D version of the video. This affects the videos at http://www.3dvisionlive.com/ and 3D videos on YouTube when viewed with a 3D set-up.
This issue has been fixed in Firefox 12
Read more about how to configure NVIDIA 3D Vision technology to work with 3D YouTube videos here.