arkenfox Firefox Hardening Guide 2026

If you want Tor Browser-grade privacy without using Tor (which is slow) or Mullvad Browser (which is great but mobile-limited), there’s a third path: harden your regular Firefox with arkenfox user.js.

arkenfox is an open-source user.js configuration that disables Firefox telemetry, hardens fingerprinting protection, and configures sensible defaults. It’s the foundation that Mullvad Browser builds on. With arkenfox, your Firefox becomes essentially equivalent to Mullvad Browser, but with Firefox features intact.

Here’s the 2026 setup guide.

TL;DR

• arkenfox is a user.js file that hardens Firefox privacy settings
• Setup time: 20-30 minutes for first-time setup
• Maintenance: Update every few months
• Trade-offs: Some site breakage. Manageable.
• Result: Privacy approaching Mullvad Browser / Tor Browser on standard Firefox

For users who want maximum privacy with Firefox features: this is the way.

What arkenfox is

arkenfox is a community-maintained user.js file for Firefox. The user.js file overrides Firefox’s default configuration with privacy-respecting alternatives.

Key features of arkenfox:
– Disables Firefox telemetry
– Disables Pocket, Mozilla services that send data
– Configures strong default tracking protection
– Disables fingerprinting-prone features (WebRTC, some sensors, etc.)
– Sets sensible search engine defaults
– Disables some performance optimizations that leak data
– Enables strict cookie isolation
– Configures secure connection defaults

It’s actively maintained on GitHub. Updated quarterly.

Why use arkenfox vs Mullvad Browser or Brave?

Different tools, different positions:

Arkenfox Firefox:
– Free, open source, configurable
– You can adjust individual settings if specific sites break
– Maintains Firefox ecosystem (extensions, bookmarks, sync if desired)
– Mobile via Firefox on Android (some user.js features work)

Mullvad Browser:
– Pre-configured (no setup time)
– Engineered as a coherent product
– Best for people who want “install and use”
– Desktop only (as of 2026)

Brave:
– Mainstream UX
– Chromium-based (not Firefox)
– Built-in ad blocking
– Trade-offs around Chromium engine and Brave’s monetization

For technical users who want control: arkenfox.

For non-technical users who want privacy without configuration: Mullvad Browser or Brave.

Installation

Step 1: Get clean Firefox

Install Firefox if you don’t have it. Use Firefox ESR (Extended Support Release) if you want the most stable arkenfox base. Standard Firefox works too.

Step 2: Locate your Firefox profile folder

macOS: ~/Library/Application Support/Firefox/Profiles/[random-string].default-release/
Linux: ~/.mozilla/firefox/[random-string].default-release/
Windows: %APPDATA%\Mozilla\Firefox\Profiles[random-string].default-release\

Easy way to find: Type about:profiles in Firefox URL bar → see “Root Directory” path.

Step 3: Download arkenfox user.js

From GitHub: arkenfox/user.js → user.js file (the main config file)

Also useful:
– prefsCleaner.bat (Windows) or prefsCleaner.sh (macOS/Linux) — cleans old prefs
– updater.bat / updater.sh — automatically updates the user.js

Step 4: Place files in profile folder

Copy user.js to your Firefox profile folder. Restart Firefox.

Step 5: Verify hardening

Open Firefox. Check that some indicators show:
– about:preferences#privacy shows “Strict” enhanced tracking protection
– about:config should show many privacy-related settings overridden
– Visit privacytests.org to verify many fingerprinting resistance features are active

Custom overrides (user-overrides.js)

arkenfox provides a default user.js. But some defaults break specific sites or features you need. For those cases, create user-overrides.js (in the same profile folder) with your custom changes.

Common overrides:

// Allow WebRTC for video calls
user_pref("media.peerconnection.enabled", true);

// Allow Notifications (some users want these)
user_pref("dom.webnotifications.serviceworker.enabled", true);

// Allow Geolocation if you trust your VPN
user_pref("geo.enabled", true);

// Enable Firefox Sync (with strong password)
user_pref("identity.fxaccounts.enabled", true);

When you run prefsCleaner or updater, your overrides persist (they’re merged with the arkenfox defaults).

What arkenfox actually changes

Major categories of changes:

Telemetry disabled

Firefox normally sends usage statistics to Mozilla. arkenfox disables this. No data flows back to Mozilla about your browsing.

Pocket and other Mozilla services disabled

Pocket is Mozilla’s “save articles for later” service. arkenfox disables it because it phones home.

Similarly, other Mozilla services (Hello, Sponsored Sites, recommendations) are disabled.

Fingerprinting resistance

Firefox has a “letterboxing” feature and font/timezone resistance options. arkenfox enables them so your browser looks more uniform.

WebRTC, which can leak your real IP even behind a VPN, is restricted or disabled by default.

Strict cookie isolation

Cookies are isolated to the site that set them (called “first-party isolation” or “total cookie protection”). Sites can’t share cookie data across each other.

Disabled features that leak data

• Battery API (used for fingerprinting)
• Geolocation (unless you opt in)
• Webcam and microphone access prompts you each time
• Push notifications disabled

Cleaner defaults

• Sets DuckDuckGo as default search (you can change)
• Disables search suggestions sent to search engines
• Disables address bar auto-fill
• Disables saved passwords (use a real password manager)

Sites that may break

Common breakage:

Video conferencing (Google Meet, Zoom, Teams): WebRTC disabled by default. Re-enable in user-overrides if needed.

Banks with aggressive anti-fingerprinting checks: Some banks see hardened Firefox as suspicious. May require Brave or unhardened Firefox.

Sites requiring geolocation: Maps, food delivery. Re-enable geo.enabled if needed.

Sites depending on third-party cookies: Some authentication flows broken. Site-specific exceptions needed.

Sites that don’t work because they specifically check for “non-Tor Firefox” fingerprint: Very rare in 2026; some news sites had this in 2020-2022.

Strategy: Live with the default arkenfox config for 1-2 weeks. Track sites that break. Add specific overrides for those sites only.

Useful supplementary extensions

Even with arkenfox, some extensions add value:

uBlock Origin — Ad/tracker blocker. Even better than browser-level blocking.

Multi-Account Containers — Isolate cookies per “container” (banking, social, shopping, etc.).

Cookie AutoDelete — Automatically clear cookies when you close tabs.

HTTPS-Only Mode is built into modern Firefox; arkenfox enables it.

Privacy Badger — Optional. Some users like it; some find it redundant with uBlock.

Search engine isolation — If you want a different search engine per container.

Don’t go overboard. Extensions are themselves fingerprinting vectors. Limit to 4-6 carefully chosen ones.

Maintenance

arkenfox is updated regularly. Quarterly is typical.

To update:

1. Run updater.bat (Windows) or updater.sh (macOS/Linux) in your Firefox profile folder
2. Restart Firefox
3. Verify everything still works

Your user-overrides.js persists through updates. You only update the base user.js.

Setting up Firefox Sync with arkenfox

Many arkenfox users initially think they can’t use Firefox Sync. You can.

Setup:

1. Disable telemetry overrides for the sync service specifically (allow only sync, not telemetry)
2. Use a strong unique password for your Firefox account
3. Enable end-to-end encryption (Firefox Sync uses it by default)

The data syncs encrypted to Mozilla’s servers; Mozilla can’t read your bookmarks/passwords/history.

In user-overrides.js:

user_pref("identity.fxaccounts.enabled", true);
user_pref("services.sync.engine.passwords", true);
user_pref("services.sync.engine.bookmarks", true);
// etc.

arkenfox on mobile

Firefox on Android: many arkenfox preferences can be set via about:config (though Mozilla has limited this in mobile Firefox). Mullvad VPN works on mobile Firefox.

iOS Firefox: Apple’s iOS rules force WebKit-only browsers, so iOS “Firefox” is actually Safari under the hood. arkenfox doesn’t fully apply.

For mobile privacy: use Firefox Focus on Android or Brave Mobile.

Performance

arkenfox can slightly slow Firefox because of all the hardening:
– Cookie isolation requires more bookkeeping
– Tracking protection inspections per request
– Disabled performance optimizations that leak data

On modern hardware: imperceptible.

On older laptops: 5-10% slower page loads vs default Firefox.

Trade-off worth making for the privacy improvement.

Common arkenfox mistakes

Mistake 1: Setting up arkenfox then never updating. Firefox and arkenfox both evolve. Old configurations become out-of-date. Update quarterly.

Mistake 2: Excessive customization. Modifying many settings can introduce bugs or break sites. Start with vanilla arkenfox, add overrides as needed.

Mistake 3: Using arkenfox without considering VPN. arkenfox protects the browser. Your ISP still sees what you visit. Pair with VPN.

Mistake 4: Confusing arkenfox with Tor. arkenfox doesn’t anonymize you against your VPN provider or your destination. For network-level anonymity, you need Tor.

Mistake 5: Forgetting to back up your user-overrides.js. If you reinstall Firefox, you lose customizations. Back it up.

Mistake 6: Disabling everything possible. Some “privacy” features (like disabling JavaScript globally) break the web. arkenfox defaults are a sensible balance; don’t push past them without understanding.

Mullvad Browser vs arkenfox: when to use which

If you want the same protection level with less setup time and less maintenance: Mullvad Browser. It’s arkenfox + Tor Project hardening pre-packaged.

If you want more configurability, regular Firefox features (sync, mobile), and you’re comfortable with config files: arkenfox.

Both are excellent. arkenfox gives you more rope (helpful and dangerous).

What we use

The Privacy Stacks team:
– 2 use arkenfox Firefox (technical preference)
– 2 use Mullvad Browser (cleaner UX)
– 1 uses Brave (mainstream privacy)
– 0 use plain Firefox (too leaky by default)
– 0 use Chrome (obvious reasons)

Disclosure

arkenfox is free and open source. Mullvad sponsors some related projects. We mention it based on quality. See our affiliate disclosure.

Last updated 2026 Q2.