Cannot watch youtube in fullscreen mode
Solution 1:
There's a fix posted here. It seems to have done the trick for me, but I'm not 100% sure it's the same problem for you. This is a problem with Flash and its hardware acceleration, not YouTube itself.
In case the above site is unavailable, here's the fix:
sudo mkdir /etc/adobe
echo "OverrideGPUValidation = 1" | sudo tee -a /etc/adobe/mms.cfg