I have the same problem in that videos are no longer playing correctly as of 3.6.6 but I am using Vista.

On youtube, videos are loading much more slowly then they were previously, I also notice that I can't play videos very well at all if I have more then one open in multiple tabs. Also, when I go to fullscreen it will either freeze up on full screen, play but I lose controls, play for a little bit and then freeze.

It's very frustrating.

What's really annoying though is that *before* that update YouTube and GoogleVideos were working fine and they now look like a child's color smears.

Having the same problem with the blurry/smeared video over here; not only with youtube but also other sites like youku, todou, etc. Plug-ins are updated, using the latest Firefox...