Cookies are invisible pieces of data that a website can store on your computer. A cookie might only be used on a single website you visit, or it may be used across many websites. The latter are called cross-site cookies, and many websites use them.
There are two main reasons websites use cross-site cookies:
- Cross-site tracking: This is by far the most common use of cross-site cookies. Trackers use cross-site cookies to collect information about the websites you visit and send them to other companies, often for advertising purposes. When you feel like an advertisement is following you around while you browse, this is a result of cross-site tracking. If the same tracker is present on multiple sites, it can build a more complete profile about you over time.
- Functional cookies: Some websites rely on these cookies in order to function properly. For example, some websites may need cross-site cookie access to let you use their service to sign in to another website (e.g., Facebook Login) or to process a payment for that website (e.g., Amazon Pay).
Firefox’s Enhanced Tracking Protection settings block cookies from cross-site trackers and isolate cookies from all other third parties. This helps prevent your browsing activity on one website from being visible to other websites. To learn more, visit Enhanced Tracking Protection in Firefox for desktop.
Managing cross-site cookies
Firefox will allow a third-party website to use cross-site cookies the first five times (or up to 1% of the number of unique sites you visit in a session, whichever is larger) without prompting you. After that, Firefox will prompt you to block these cookies. Firefox blocks these cookies at that point because a site requesting access that many times may be a tracker.
Third-parties will only be able to prompt you if you interact with the website you are on. For example, if you visit dogs.com, and select the payment field, Amazon Pay cross-site cookies may be allowed to facilitate that transaction. After that, Firefox will ask you if you want to keep allowing them.
From the site information panel, you can click the X next to a site to revoke previously accepted access.
If a third-party continues to use cross-site cookies across multiple sites, this becomes a signal to Firefox that the third-party might be a tracker. At that point, the third-party would have to prompt you to ask for permission to use cross-site cookies.
Firefox also automatically grants access to a number of third-parties for website compatibility.
Third-party trackers are invisible snippets of code that are present on many websites. They collect and send information about your browsing history to other companies, often for advertising purposes. If the same third-party tracker is present on multiple sites, it can build a more complete profile about you over time. When you feel like an advertisement is following you around while you browse, this is a result of third-party trackers.