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I'm having the same problem. I can only get the HTML5 videos to play on YouTube. The same flash videos that don't work directly on YouTube do work when they are embedded in other sites, however. Other flash content works fine.
 
This has been going on for about a month with my computer. Everything is up to date. I've been using Chrome, but would like to switch back.
 
Same, I'm having the same problem for a month or so. Every other flash content works except youtube videos.

Though if they are embedded or in a channel, I can see them. And I don't wanna use chrome or firefox, use more cpu while watching a flash compared to safari :(
 
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