For long I have been having problem with flickering flash videos in the Firefox 4 beta in Ubuntu 10.10 64-bit. Today I found a solution, and I have no longer any reason to use Chrome. 
It seems that the problem with flickering flash videos only occurs with Adobe Flash Player 10.1. So, by installing the beta 10.3 of Flash Player, the problem is now long gone.
I don’t know how you install the Flash beta in the 32-bit version of Ubuntu. But in the 64-bit version you just need to type these lines in the terminal:
sudo add-apt-repository ppa:sevenmachines/flash sudo apt-get update sudo apt-get install flashplugin64-installer
I hope this solution works for you guys!
I have two monitors to my computer, and I like to have videos in fullscreen on my left monitor. But if you click outside of the video (on the second monitor) does the video goes back to its original size. Lucky for us has a guy came up with an idea to solve this. This solution can be found on this website:
Apparently changes Adobe Flash Player 10.1 my graphic card’s clock speed to the default clock when I start watching a YouTube-video. The graphic card that I have is a ATI Radeon PowerColor 4870 and the reason why this is a problem is because my card is kinda broken and I need to down clock it to not get strange dots all over the screen. I guess I have to download Flash Player 9 if I can’t find an alternative or a solution to my problem. I could buy a new graphic card, but I don’t know if I want to do that since this is working just fine when it’s down clocked a bit.