I was having a problem with flash 11.2 as well, so I tried flash 10.3 and it works.
download.macromedia.com/pub/flashplayer/installers/archive/fp_10.3.183.43_archive.zip

This is an older laptop PIII so that's probably the reason why...
flags: fpu vme de pse tsc msr pae mce cx8 sep mtrr pge mca cmov pse36 mmx fxsr sse

In the latest (beta) driver 310.14, nvidia ported the patch on his drivers, so without workarounds or external libvdpau there are no more blue videos on youtube.

edit: 310.19 are no more in beta.

Wow, I guess I can finally stop beating my head against the wall. I tried everything I could think of to get the newest flash working, and could not understand why only 10 worked. Thank you for ending this confusion for me!

I didn't have the sse2 flag and Flash 11 did not work for me.
Fir0n had both sse and sse2, Flash 11 didn't work for him.

So is Flash 10 needed if you only have sse? It would make sense for my older computer, but why is Flash 11 working for Don but not for Fir0n?

@Beelzebud You are welcome.

@zaphar If 'sse2' isn't shown, Flash 11 won't work. If Fir0n would explain his case in details, we could know what's going on in his particular situation.

Theres a html5 trial happening for youtube http://www.youtube.com/html5?gl=GB&hl=en-GB might be of help, other than that try enabling/disabling hardware accelaration in flash (right click on any video for the options) it might help you get working video.

fir0n,i'm not sure, but after copying #cp libflashplayer.so /usr/lib/mozilla/plugins/

try this (maybe you forgot to do it) -> chmod +x /usr/lib/mozilla/plugins/libflashplayer.so
restart FF,should work

found this thread while my slackbuild loops

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Joy