How Firefox Blocks Fingerprinting Using Disconnect Lists

This article explains how the Mozilla Firefox web browser protects user privacy by detecting and blocking known browser fingerprinting scripts. By integrating the open-source Disconnect tracking protection list into its Enhanced Tracking Protection (ETP) engine, Firefox identifies domains known to harvest unique device configurations. When a user visits a webpage, Firefox cross-references incoming third-party script requests against this local list and terminates any matching connections before the fingerprinting code can download or execute.

Understanding Browser Fingerprinting

Browser fingerprinting is a tracking technique that collects specific details about a user's device and browser configuration. Instead of relying on traditional cookies, fingerprinting scripts query browser APIs to gather data such as screen resolution, installed fonts, operating system, battery level, and graphics card rendering capabilities (Canvas and WebGL). Combined, these data points create a unique profile—or "fingerprint"—that can track a user across different websites, even in private browsing modes.

The Disconnect Protection List

Rather than analyzing the behavior of every script in real time—which would severely slow down web browsing—Firefox relies on a blocklist maintained by Disconnect, a privacy software company.

Disconnect curates an open-source list of known tracking companies and categorizes the domains they use. To classify a domain as a fingerprinter, Disconnect uses automated crawlers and manual audits to observe which scripts actively query high-entropy browser APIs for tracking purposes. Domains confirmed to engage in pervasive fingerprinting are placed into a specific category within the Disconnect list.

How Firefox Detects Fingerprinting Scripts

Firefox integrates the Disconnect list directly into the browser using the following mechanism:

  1. Local List Updates: Firefox automatically downloads the latest Disconnect protection lists from Mozilla’s servers and stores them locally on the user's device. This ensures Firefox does not need to send user browsing data to a remote server to check if a site is safe.
  2. Monitoring Network Requests: When a user navigates to a website, that website begins making network requests to load images, stylesheets, and external JavaScript files.
  3. URL Matching: Before Firefox allows a third-party network request to go through, its internal URL Classifier checks the destination domain against the locally stored Disconnect fingerprinting list.

How the Blocking Process Works

If the URL Classifier detects a match between a third-party script request and an entry on the Disconnect fingerprinting list, Firefox takes immediate action at the network layer:

  • Connection Termination: Firefox cancels the HTTP/HTTPS request entirely. The connection to the fingerprinting server is never established.
  • Preventing Execution: Because the network request is blocked, the fingerprinting JavaScript file is never downloaded to the browser. Consequently, the script cannot query the browser's APIs, read device attributes, or transmit a fingerprint back to the tracker's server.
  • Silent Failure: The browser handles the blocked request silently so the rest of the webpage can continue loading normally without crashing, preserving the user experience.

Balancing Protection and Website Functionality

Some scripts use fingerprinting techniques for legitimate security purposes, such as fraud detection during banking or bot prevention during logins. Blocking these scripts can break essential website functions.

To address this, Disconnect separates scripts that fingerprint strictly for cross-site advertising/tracking from those that provide security services. Firefox specifically blocks domains on the Disconnect list categorized as participating in invasive cross-site tracking, minimizing the chances of breaking legitimate website features. This protection is enabled by default in Firefox's "Standard" and "Strict" Enhanced Tracking Protection settings.