High CPU usage while playing Flash

I am just guessing here since Flash Player is a buggy binary blob – but high CPU load could mean that Flash Player is not using hardware acceleration. So, go to any YouTube video, open it in fullscreen (this is important due to weird bug in Flash Player in case you're using Unity 3D / Compiz), right click on video, select Settings… and make sure that Enable hardware acceleration is checked.

You could also try to delete Flash player's settings, you may however lose some "important" data like checkpoints in Flash games (data exclusively stored by Flash Player). Quit browser and delete/move away ~/.macromedia/Flash_Player directory with Nautilus or using this command:

rm -r ~/.macromedia/Flash_Player

Also take a look if you have libvdpau1 package installed, this allows video players to use hardware video decoding. Flash Player uses this to some extent too, but it's buggy (under some conditions, videos on YouTube may have swapped blue and red colour channels). See this answer for more details.

reduce buffer manually to 10 mb and you are good to go. (its a setting in Advanced - Network - buffer settings) Restart Firefox and use 'top' to view reduced cpu use. Mine went from 60 to 6% and process ' plugin container (flash) went from 45-50% to 30%

I use Ubuntu 12.04, 64 bit on dual core 2.4 GHZ and 3GB ram

Happy browsin'