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Yeah, stopped working here too. Probably Youtube changed something in the way they embed the flash video. Let's hope the author of Youtube5 can push an update for us soon!
 
I'm on Lion 10.7.4 with the latest Safari and it's been doing the same thing in the past few days.
 
It is a useful extension, however, I switched to using clicktoflash a while back. It provides the same YouTube ability, but with a much more useful player. It also supports many other video websites.
 
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