Comment 16 for bug 225165

These bug reports maybe related and handy as well:
https://bugs.launchpad.net/ubuntu/+source/human-theme/+bug/293637
https://bugs.launchpad.net/ubuntu/+source/xorg-server/+bug/294972
https://bugs.launchpad.net/ubuntu/+source/firefox/+bug/89267
https://bugs.launchpad.net/ubuntu/+source/xulrunner-1.9/+bug/323587

Also using a flash blocker in the browser solved the problem partially for me in the sense that you are not loading flash when doing normal browsing. However this doesn't solve the problem for the WebM (html5) codec (Firefox can't play the h.264 as far as I can tell). There are other tips on the net how to disable video plugins for Firefox, not sure how wished for, or easy to use this is. Disabling all plugin use in Firefox, cause fallback behavior where the plugin is played in the browser itself, thus transferring the problem back to the browser.

This also happens in chromium.

Browser and versions (from stable repo's):
Firefox: 10.0.2
Chromium: 17.0.963.56~r121963
Epiphany: 3.0.4-1

(Side note: Trying to fill in this bug report I had a 'race' condition in firefox of 80/100% cpu and 580 mb of mem used with 6 tabs open. Restarting Firefox and closing the tab for youtube (no flash installed), and turning off javascript for launchpad.net and ubuntu.com solved the problem.