Umm... Cookies from different sites are already separate. A cookie is set for a particular domain and your browser only allows servers in that domain to read it

Umm... Cookies from different sites are already separate. A cookie is set for a particular domain and your browser only allows servers in that domain to read it. You don't have to worry about some random site reading a cookie set by your bank's website. What may be of concern are the tracking and analytics cookies. Let's say you visit a site that displays ads served up by a third party. The ad firm sets a cookie with a unique identifier. Later on you visit some other site that displays an ad from the same ad firm. The ad firm is able to read the cookie they set earlier. Now you're being tracked (not personally, but as a unique web user) and ads can be tailored to your browsing habits.