Hopefully you already have off-domain cookies turned off, but if not then do that.
Unless I'm mistaken, the concept of an off-domain cookie is this: when I visit example.com, cookies may be stored from example.com and ads.otherserver.com, assuming example.com uses that site for some purposes.
I already use Firefox with private browsing permanently enabled, noscript, and ghostery, among others (although the latter isn't set to disable all cookies). Are off-domain cookies already disabled? If not, how can I disable these off-domain cookies?
EDIT: Just to clarify, my Firefox privacy settings are currently set to this: