Just as a heads-up for others who might be having the same problem: if you're using Firefox and Youtube recently stopped working for the most part (i.e., you can still watch videos, but not leave comments, use channel pages etc.), and if your error console (Ctrl-Shift-J) shows errors such as this...
Error: uncaught exception: [Exception... "Security error" code: "1000" nsresult: "0x805303e8 (NS_ERROR_DOM_SECURITY_ERR)" location: "http://s.ytimg.com/yt/jsbin/www-core-vfl0u_k4M.js Line: 227"]
...then head to about:config and check if dom.storage.enabled is set to false. If it is, change it back to true; that should fix it. The same applies if you've been having trouble composing emails in GMail.
If you're concerned about long-term tracking using DOM storage, install the BetterPrivacy add-on and have it delete DOM storage data automatically.