Simon at InterWeb Task Force writes about the hidden tracking that the Ant Addon for FireFox is doing and the repercussions of it:
What if the US government subpoena ‘Reality check network corp’ for all information stored on their servers about my IP address, cooke, or UID? Lets think even more simplistic, what if a party to a divorce case subpoenas for that data to prove a partner was visiting certain sites at certain times? Assuming this data is recorded by ant.com on their server rpc.ant.com in New York (and lets face it, why would they send such data with unique identifiers if it were not recorded?), my entire browsing history is there laid before the subpoenaing court or government. Every site I visited. Every page I looked at.
He admits that this is likely done to improve a ranking algorithm they use, but the fact that they are using a unique ID for users is a touch concerning, or at least it should be. Anyways Chrome and Safari are much better options.