Anti-Tracking Guide

Firefox fingerprinting protection

What Firefox's Known and Suspected Fingerprinters protections do, what they can break and where to switch them on, based on Mozilla Support.

Official source: Mozilla Support, Firefox's protection against fingerprinting. Verified on

1 min read · Review scheduled for 2027-01-19

Contents

Two protections

Firefox uses two protections, both part of Enhanced Tracking Protection (Mozilla):

  • Known Fingerprinters blocks scripts that appear on Disconnect's fingerprinting list.
  • Suspected Fingerprinters limits the information exposed to websites for scripts Firefox cannot block.

What Suspected Fingerprinters changes

Mozilla lists these measures (Mozilla):

  • Adds noise to canvas images when a site reads them back.
  • Ignores locally installed fonts that are not standard for your operating system.
  • Reports touch-point counts as 0, 1 or 5.
  • Reports CPU cores as either 4 or 8.
  • Reports the available screen height with a fixed subtraction.

Defaults

Known Fingerprinters is on in normal browsing and in Strict mode. Both protections are on in Private Browsing and in Strict. The Custom setting lets you control each one (Mozilla).

How to check or change it

  • One site: click the shield icon beside the address bar and switch protection off for that site. Other sites stay protected.
  • All sites: open Settings > Privacy & Security, choose Custom under Enhanced Tracking Protection and switch on Known and Suspected fingerprinters (Mozilla).

What it can break

Mozilla lists custom fonts that are not installed by default, emoji detection, some image or video effects, some glyphs in certain scripts, multi-device touch input, window sizing and slower complex calculations. If a site breaks, switch protection off for that site to test (Mozilla).

Limits

Known Fingerprinters only blocks companies on the list, so it does not stop all fingerprinting. Mozilla sometimes adds exceptions to keep sites working, which can reduce protection. Suspected Fingerprinters cannot cover every method (Mozilla).

Outside this page: We describe Firefox only because the source we read is Mozilla's. We make no comparison with other browsers.