Firefox 85 to Feature Enhanced Tracking Protection

The Mozilla development team plans to introduce a new network partitioning mode in Firefox 85 to block tracking methods that follow users across websites using identifiers stored in areas not intended for permanent data storage, known as “supercookies.”

How Supercookies Work

One type of supercookie used by tracking systems relies on checking for the presence of certain data in browser caches. For example, the existence of previously loaded images, fonts, certificates, stylesheets, and other cached data can serve as flags for tracking. Currently, all cached resources are stored in a shared namespace, regardless of the original domain. This allows one site to determine if resources from another site have been loaded by checking the cache.

Network Partitioning as a Solution

The new network partitioning protection will add an extra attribute to the key used for retrieving objects from browser caches. In addition to the URL, the cache will now be tied to the primary domain from which the main page was opened. This limits the cache’s scope for tracking scripts to the current site only, preventing scripts in iframes from checking if a resource was loaded from another site.

Industry Adoption and Trade-Offs

A similar technique has been used in Safari since 2013 and is planned for inclusion in Chrome. The downside of this approach is reduced caching efficiency, as network caches can no longer be shared across different sites. Before fully enabling network partitioning in Firefox 85, Mozilla plans to conduct experimental testing to assess any potential negative impact on performance.

How to Manage the New Protection

You can control the new protection method in Firefox by adjusting the privacy.partition.network_state setting in about:config.