How to disable screen saver when watching a movie on youtube?

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http://askubuntu.com – I have managed to stop screen saver with vlc. But it seems that when I am watching a movie on firefox the screen saver pops in again, in a few mins. How can I tell my ubuntu 12.04 that I am watching a movie in firefox and not to turn on the screen saver each time I do so? (HowTos)