Privacy is a human right, and is core to Mozilla’s mission. However many companies on the web erode privacy when they collect a significant amount of personal information. Companies record our browsing history and the actions we take across websites. This practice is known as cross-site tracking, and its harms include unwanted targeted advertising and divisive political messaging.

Last year we launched Enhanced Tracking Protection (ETP) to protect our users from cross-site tracking. In Firefox 72, we are expanding that protection to include a particularly invasive form of cross-site tracking: browser fingerprinting. This is the practice of identifying a user by the unique characteristics of their browser and device. A fingerprinting script might collect the user’s screen size, browser and operating system type, the fonts the user has installed, and other device properties—all to build a unique “fingerprint” that differentiates one user’s browser from another.

Fingerprinting is bad for the web. It allows companies to track users for months, even after users clear their browser storage or use private browsing mode. Despite a near complete agreement between standards bodies and browser vendors that fingerprinting is harmful, its use on the web has steadily increased over the past decade.

We are committed to finding a way to protect users from fingerprinting without breaking the websites they visit. There are two primary ways to protect against fingerprinting: to block parties that participate in fingerprinting, or to change or remove APIs that can be used to fingerprint users.

Firefox 72 protects users against fingerprinting by blocking all third-party requests to companies that are known to participate in fingerprinting. This prevents those parties from being able to inspect properties of a user’s device using JavaScript. It also prevents them from receiving information that is revealed through network requests, such as the user’s IP address or the user agent header.

We’ve partnered with Disconnect to provide this protection. Disconnect maintains a list of companies that participate in cross-site tracking, as well a list as those that fingerprint users. Firefox blocks all parties that meet both criteria [0]. We’ve adapted measurement techniques from past academic research to help Disconnect discover new fingerprinting domains. Disconnect performs a rigorous, public evaluation of each potential fingerprinting domain before adding it to the blocklist.

Firefox’s blocking of fingerprinting resources represents our first step in stemming the adoption of fingerprinting technologies. The path forward in the fight against fingerprinting will likely involve both script blocking and API-level protections. We will continue to monitor fingerprinting on the web, and will work with Disconnect to build out the set of domains blocked by Firefox. Expect to hear more updates from us as we continue to strengthen the protections provided by ETP.

[0] A tracker on Disconnect’s blocklist is any domain in the Advertising, Analytics, Social, Content, or Disconnect category. A fingerprinter is any domain in the Fingerprinting category. Firefox blocks domains in the intersection of these two classifications, i.e., a domain that is both in one of the tracking categories and in the fingerprinting category.