I've experianced the same behaivour with some youtube vids and the link you posted in the new package thread. I also noticed that the problem did not exist on VL6.0 light Live RC1 untill I installed the Nvidia drivers.
Anyhow to cut a long story short (much fiddling with options in xorg.conf) I finally found that dissabling 'hardware acceleration' in Flash settings solves the problem on my GF 6600GT. According to the Adobe site HW acceleration reduces cpu load when in full screen by offloading scaling. My cpu (amd xp3200, 2.2GHz) seems fine at full screen.
I know this isn't a real solution but at least it provides a work around for some although I I'm left wondering where the problem lies, Nvidia, Flash or firefox?
Thank you so much!
dissabling 'hardware acceleration' in Flash settings solved the problem.