Mozilla Implements Data-Driven Protections to Reduce Browser Fingerprinting

Mozilla has integrated new fingerprinting protections into Firefox that significantly degrade the precision of tracking scripts by reducing the unique identifying details a browser exposes. By analyzing data from nearly 25 million users, researchers identified specific browser features that, when generalized, make individual users look more like one another to trackers without breaking website functionality.

Identifying the Core of Fingerprinting

Browser fingerprinting is a technique where trackers combine ordinary details—such as available fonts, screen dimensions, and graphics hardware—to build a uniquely recognizable profile of a user. This allows websites to track individuals across different sessions even when they do not use cookies.

Mozilla's research focused on identifying which specific details contribute most to making users uniquely identifiable. By simulating measurements from major tracking scripts like Akamai and Signifyd, the team determined that protecting a small handful of features could significantly weaken popular fingerprinting techniques.

“Our goal is to make more users’ browsers look similar to fingerprinting scripts while preserving the information websites need to function correctly, like language and timezone settings.”

— Open research, stronger privacy: Making Firefox users harder to fingerprint – Mozilla Security Blog

Technical Mitigations and Standardization

To reduce the uniqueness of a browser's profile, Firefox now limits the information exposed through screen resolution APIs. This prevents trackers from using the specific size and position of a user's system taskbar or dock as a distinguishing factor.

The browser also standardizes hardware reporting by providing generic processor counts. Additionally, Mozilla addressed inconsistencies in image rendering and Web Audio outputs, which previously provided unique signals that could be used to identify specific devices.

Deployment and Privacy Trade-offs

Mozilla is deploying these protections using a tiered strategy to balance privacy with site compatibility. Some measures are active by default in all browsing modes, while others are initially restricted to Private Browsing or 'Strict' Enhanced Tracking Protection (ETP) mode.

The goal of these changes is to make users look similar to one another while preserving essential information like language and timezone settings. However, Mozilla notes that these measures do not make fingerprinting impossible, as trackers can adapt to new defenses over time.