I get serious slowdown with multiple (3--5) youtube tabs open in firefox, but not chrome. Seems to happen when tabs are open for a long time (weeks), so probably some leak. Lots of others mention it on forums.
It isn't unique to youtube either. Gmail offline mode only works on chrome, even though other browsers have the necessary APIs. And menu copy and paste in google docs uses a special chrome-only extension that google pre-installs in chrome, instead of the clipboard API that works in other browsers as well.
