Firefox Resource Reader

1. What is the Firefox Resource Reader in Fingerprinting?

The resource:// URI scheme in pre-Quantum Firefox (versions <57) allows access to internal browser files, including scripts, configurations, and localization data, even from ordinary web pages via <script> tags.

Although Firefox never intentionally exposed these files to fingerprinting, poor isolation made it possible for malicious scripts to fetch internal assets like:

• Locale & versioning files (firefox-l10n.js, greprefs.js)
• Build metadata (branding, build type, Tor detection flags)
• Default preferences (sync settings, PDF.js configuration)

This old Firefox quirk allowed websites to extract unique browser details, potentially compromising user privacy if combined with other fingerprinting techniques.

2. How Platforms Historically Detected Firefox Resource Fingerprints

Before Firefox Quantum (2017), scripts could exploit the resource:// scheme to identify:

1. Platform & Locale Detection

• Accessing files like firefox-l10n.js could reveal the system’s default language (en-US, fr-FR, etc.).
• Non-standard locale formats (e.g., es-AR vs. es-ES) could hint at custom Firefox builds.

2. Firefox Build & Version Fingerprinting

• Files like firefox-branding.js or greprefs.js exposed whether the browser was:
  • Official Build (standard release)
  • ESR (Extended Support Release)
  • Beta/Nightly versions
  • Tor Browser (via absence of updates)

3. Default Configuration Leaks

• Scripts could extract hashes of preference files (services-sync.js, webide-prefs.js), detecting unusual modifications or custom Firefox forks.
• Missing files (e.g., 000-tor-browser.js) could be checked to infer Tor Browser usage.

This technique became obsolete after Firefox Quantum (v57+) due to stricter resource isolation.

3. How FlashID Handles Legacy Firefox Resource Fingerprinting

Since modern FlashID environments primarily use up-to-date browsers, legacy Firefox vulnerabilities are not applicable by default.

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