Note that in Firefox, a google.com "PREF" cookie, which Google says is meant to save language preferences and the like, will from time to time suddenly be set, even if you have only a blank tab open.
It's not set by any website you happen to be visiting - it's Google who's setting these cookies. They are saved even if you don't have any webpage open!
This behaviour has been known for some months and concerns have been expressed about it, as it could conceivably do more than Google says it does.
In Firefox 13, even after deleting all cookies, turning off Firefox's New Tab page and disabling Safe Browsing, I found that this cookie kept re-appearing. So the previous fix of disabling Safe Browsing in order to stop this cookie no longer works in Firefox 13, from my testing yesterday.
As for the Chrome browser, although a few months ago Chrome did not automatically set this cookie, the Attacat Cookie Tool kept reporting Google cookies ("NID" and "PREF") even when only a blank tab was open and no cookies were visible via Chrome's settings page! So perhaps it's now impossible to prevent these cookies in Chrome too. (This could be an issue with Attacat's tool, though; I'll report it to them.)
However, it seems Internet Explorer doesn't get any PREF cookies, for now. I haven't tested it in Opera yet.
So - should there be a cookie law notice & consent for the PREF cookie? And who should be responsible for that?