As much as you guys are right in term of user's privacy, there are a few points to remember :
- data is anonymous. There is no easy way to retrieve all keys from Memcache anyway. Its only purpose is to imitate the cookie's behavior. It won't store more, it won't store less.
- I strongly agree that users should be able to deny third party cookies or cookies in general if they want. That is their right to privacy. However, Firefox's move is going to kill the business of thousand of companies all around the world (ad networks and traffic analysis companies) and I believe that is an selfish move on their part. You should think about other people when you develop these types of tools and activate them by default.
It's not normal to stumble blindly into them. It's not like people wake up one morning with randomly assigned jobs. Depending on the kind of job, it's also normal for everyone else to defend themselves against that job. Avoid such jobs.
