I use Facebook, but I don't want them to know which other sites I visit. I've seen it happen, was logged in to FB, then visited another website to order something, and now I see the ads from that website suddenly appear in FB. Every site that has a FB button, loads that button from FB, so they know which sites I visit.
Firefox is my primary browser. For the moment I've solved this by using Safari for Facebook only, but I would prefer to use Firefox all the time.