If you installed Ubuntu Maverick on a old computer with Intel graphics chipset, you will find that you won’t be able to watch YouTube video in full screen, regardless which browser you are using. Apparently, the problem lies with the embedded Flash player conflicts with the hardware acceleration.
For those who are having this problem, here’s the fix:
1. Disable hardware acceleration
1. Open your browser and go to YouTube.com. Start playing your video.
2. Right click on the video and click Settings.
3. On the flash popup, uncheck “Enable Hardware Acceleration”. Click Close.
Restart your browser and play the video in full screen mode.
Overriding GPU validation
If the above trick does not work, here’s what you can do:
Open a terminal and type the following command:
sudo mkdir /etc/adobe
echo "OverrideGPUValidation=true" >~/mms.cfg
sudo mv ~/mms.cfg /etc/adobe/
Start your browser and play the video in full screen. It should work now.
If you are using Firefox, you can install the FlashVideoReplacer extension. It replaces the flash player with movie player and is able to play the video in full screen and in HD quality.
via Ubuntu Forum