dstiles - 9:51 pm on Mar 16, 2013 (gmt 0)
For Firefox I recommend installing a cookies editor add-on (I also ALWAYS include NoScript to improve my chances, even though I use Linux). I find it quite easy to set up cookie handling: first setup of FF I set, in the Privacy pane, "Do not accept 3rd party" and "ask me every time" - the latter is slightly annoying but at least I know what I'm accepting.
Since youtube is a google site I would not be surprised if G used that excuse to set G-global cookies: unethical but hey, it's big G.
Possibly similar with piwik, if the piwik package itself sets the cookie. I think advertisers get away with a similar thing IF you let them through at all.