Firefox Privacy Features: A Technical Deep Dive for Developers

Developer tools in the browser are commonly known. Less commonly known: the privacy-focused features that Firefox ships by default that Chrome either doesn't have or added later and less completely.

Here's a technical breakdown for developers and privacy-conscious users.

Enhanced Tracking Protection (ETP)

Firefox ships with Enhanced Tracking Protection on by default. It blocks:

• Social trackers — Facebook, Twitter, LinkedIn tracking pixels on other sites
• Cross-site tracking cookies — third-party cookies that track you across sites
• Fingerprinting scripts — scripts that attempt to build a fingerprint from browser properties
• Cryptominers — scripts that use your CPU for cryptocurrency mining

The blocklist comes from Disconnect.me's tracking protection list. You can see what's been blocked on any page via the shield icon in the address bar.

What this means for developers: If you're testing analytics or third-party integrations, Firefox's default ETP may block them. Test in a private window or temporarily disable ETP for localhost — Firefox lets you do per-domain exceptions.

Total Cookie Protection (TCP)

Introduced in Firefox 86, enabled by default since Firefox 103. TCP puts each website's cookies in a separate "jar" — a third-party cookie from site A loaded on site B cannot be read by site A on site C.

This is a more thorough fix than blocking third-party cookies entirely (which breaks some legitimate cross-site functionality). TCP maintains the functionality while preventing the cross-site tracking.

Technical detail: TCP is implemented at the cookie storage level, not the network level. Cookies are still sent and received as normal — they're just isolated per first-party context.

DNS-over-HTTPS

Firefox was the first major browser to ship DoH by default (in the US), routing DNS through Cloudflare's 1.1.1.1 or NextDNS. DoH encrypts DNS queries that would otherwise be visible to your ISP or network admin.

Configuration: Settings → Privacy & Security → DNS over HTTPS.

You can set a custom DoH provider (Quad9, NextDNS, etc.) for more control.

Fingerprinting Resistance

Firefox ships with fingerprint resistance features that Chrome doesn't have natively:

• privacy.resistFingerprinting (about:config) — randomizes or standardizes Canvas, WebGL, AudioContext, fonts, screen size, and time zone to make fingerprinting harder
• Letterboxing (when resistFingerprinting is enabled) — adds gray bars around the viewport to prevent window size from being used as a fingerprint signal

These are the same features used in the Tor Browser (which is based on Firefox ESR).

Extension Signature Enforcement

Firefox requires extensions to be signed by Mozilla's AMO review team (with a few exceptions for enterprise deployments). This prevents unsigned, unreviewed extensions from being installed.

Chrome dropped similar protection in Manifest V2; most malicious extension campaigns targeted Chrome. Firefox's requirement adds a meaningful review step — not foolproof, but it raises the bar.

Firefox Monitor Integration

Firefox's built-in Firefox Monitor integration (via the Firefox Account toolbar) can alert you to data breaches affecting your email address. It uses the haveibeenpwned.com database via Mozilla's relay infrastructure (Mozilla doesn't see your email in plaintext).

What Firefox Doesn't Have: Privacy Sandbox

Google's Privacy Sandbox is Chrome's replacement for third-party cookies — an attempt to enable targeted advertising without cross-site tracking cookies. Firefox has explicitly not implemented Privacy Sandbox, on the grounds that the Topics API (a Privacy Sandbox component) is still a tracking technology, just implemented differently.

For users who don't want any browser-level participation in advertising infrastructure: Firefox is the default-safe choice.

The Developer's Checklist

If you're building a web app and want to ensure it works for privacy-conscious Firefox users:

1. Test with ETP enabled in Firefox — check whether any features break
2. Test with Firefox's third-party cookie isolation — do your auth flows still work?
3. If you're using Canvas or WebGL for fingerprinting: it won't work reliably in Firefox with resistFingerprinting enabled
4. If you're using third-party tracking scripts: assume they're blocked for a meaningful percentage of Firefox users
5. Check your CSP — Firefox's stricter parsing has caught CSP misconfiguration that Chrome allowed

Privacy-respecting extensions like Weather & Clock Dashboard don't use tracking, don't make cross-site requests, and work correctly regardless of ETP or TCP settings. That's the design goal: an extension that a privacy-conscious user can install without inspection.