If you have problems with watching videos in fullscreen (youtube or other)
Go to terminal and type:
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sudo apt-get remove swfdec-mozilla mozilla-plugin-gnash adobe-flashplugin flashplugin-nonfree
Now install flashplugin-nonfree
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sudo apt-get install flashplugin-nonfree
Now create new directory /etc/adobe
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sudo mkdir /etc/adobe
Create new config
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echo "OverrideGPUValidation=true" >~/mms.cfg
Them move this config file to created folder
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sudo mv ~/mms.cfg /etc/adobe/
Restart firefox and go to youtube and play video in fullscreen
Unique, jesi lik stvarno 
