First try this. Disable hardware acceleration from within any Flash video window. Right click the video area, select "Settings", then remove the tick for "Enable hardware acceleration". Just once will change it for all browsers.
If that doesn't work, uninstall Flash using Adobe's own Flash uninstaller, reboot the computer, then revisit Adobe's web site to reinstall Flash.
Here is Adobe's Flash uninstaller - http://helpx.adobe.com/flash-player/kb/uninstall-flash-player-windows.html
You don't say if this computer is Mac, Linux, or Windows. The options above are for Windows and may be different for other OSes.
It happens on google chrome and on mozilla, even the manus that pop up when you right click come out pixelated.
What should I do?