On UBUNTU 11.04 x64 and Firefox 4 when I play flash videos there are several white squares flashing around. If I hit the Full-Screen then all “blocks” goes away. I’m running with Flash Player 10.3.181.14.
Switching to Flash Player x64 solve the problem.
Download from http://labs.adobe.com/technologies/flashplayer10/square/ 64bit flash plugin for Linux and put libflashplayer.so in ~/.mozilla/plugins/