I have recently been having a problem with my web browsers (Firefox and Chromium) playing all videos (Youtube and ect...) I have uninstalled flash and then re-installed it and done the same to my web browsers, but none of these have seemed to have worked any thought oor ideas to help will be greatly appreciated.
I'm on a 2009 MacBook Pro 17" and had the same problem, until I found this forum thread.
It says go to a video in a web page, right click on Settings, turn OFF Hardware Acceleration. Reload the page and play. :)