ContainSite

Per-site container isolation with unique device fingerprints for Firefox and LibreWolf.

Every website you visit is automatically placed in its own isolated container with a unique, deterministic device identity. Sites cannot share sessions, cookies, or correlate you through browser fingerprinting.

What it does

• Automatic per-site containers — each domain gets its own container on first visit, no configuration needed
• Unique fingerprints per container — every container presents a completely different device to websites
• Auth-aware — login redirects (e.g. YouTube to Google) stay in the originating container so authentication works seamlessly
• Cross-site navigation — clicking a link to a different domain automatically switches to the correct container
• HTTP header spoofing — User-Agent, Accept-Language, and Client Hints headers match each container's identity
• Configurable — toggle individual fingerprint vectors, whitelist domains, manage containers from the options page
• Zero configuration — install and browse, everything is automatic

Fingerprint vectors protected

Vector	Method
Canvas	Deterministic pixel noise per container seed
WebGL	Spoofed GPU vendor, renderer, max parameters, and normalized extensions
AudioContext	Seeded noise on frequency and channel data
Navigator	CPU cores, platform, languages, device memory, oscpu
Screen	Resolution, color depth, window dimensions
Timezone	getTimezoneOffset, Date.toString, Intl.DateTimeFormat
WebRTC	Forced relay-only ICE policy (blocks local IP leak)
Fonts	Noise on measureText + DOM element dimensions (offsetWidth/Height etc.)
Font API	document.fonts.check() blocked, size reports 0
ClientRects	Sub-pixel noise on getBoundingClientRect
Plugins	Reports empty
Battery	Always reports full/charging
Connection	Fixed network profile
HTTP Headers	User-Agent, Accept-Language spoofed per container; Client Hints stripped
Speech Synthesis	getVoices() returns empty, voiceschanged suppressed
matchMedia	Screen dimension queries return spoofed values
Performance	performance.now() precision reduced to 0.1ms
Storage	navigator.storage.estimate() returns generic values

How it works

1. You visit youtube.com in a normal tab
2. ContainSite creates a youtube.com container and reopens the tab in it
3. A deterministic fingerprint is generated from a random seed and injected via exportFunction() before any page scripts run
4. You visit gmail.com — gets its own container with a different fingerprint
5. YouTube and Gmail cannot share cookies, sessions, or device identity

When YouTube redirects you to accounts.google.com for login, the redirect stays in YouTube's container. Gmail has its own separate Google login in its own container. Same authentication flow, fully isolated identities.

Architecture

Background Script
  ├── Auto-creates containers per domain (contextualIdentities API)
  ├── Generates deterministic fingerprint from seed (Mulberry32 PRNG)
  ├── Registers per-container content scripts (contentScripts.register + cookieStoreId)
  ├── Intercepts navigation to assign tabs to containers
  └── Spoofs HTTP headers (User-Agent, Accept-Language, Client Hints) per container

Content Script (per container, ISOLATED world, document_start)
  └── Uses exportFunction() + wrappedJSObject to override page APIs
      ├── Canvas, WebGL, AudioContext prototypes
      ├── Navigator, Screen, Performance properties
      ├── Timezone (Date, Intl.DateTimeFormat)
      ├── WebRTC (RTCPeerConnection)
      ├── Font metrics (measureText, DOM dimensions, document.fonts)
      ├── ClientRects, Battery, Connection, Storage
      ├── Speech synthesis, matchMedia
      └── Plugins, mimeTypes

Uses Firefox's exportFunction() API to inject overrides from the isolated content script world directly into the page context. This bypasses Content Security Policy restrictions that block inline script injection.

Install

From file

1. Download the latest .xpi from Releases
2. In Firefox/LibreWolf: about:addons → gear icon → "Install Add-on From File..."
3. Select the .xpi file

For unsigned installs, set xpinstall.signatures.required to false in about:config (LibreWolf has this off by default).

From source

1. Clone the repo
2. Go to about:debugging#/runtime/this-firefox
3. Click "Load Temporary Add-on..."
4. Select manifest.json

Popup UI

Click the ContainSite toolbar icon to see all active containers. From there you can:

• Toggle fingerprint spoofing on/off per container
• Regenerate a container's fingerprint (creates a new device identity)
• Prune Unused — remove containers with no open tabs
• Reset All — clear all containers and data

Options Page

Right-click the toolbar icon → Manage Extension → Preferences to open the full options page.

Fingerprint Vectors

Toggle individual spoofing vectors on or off globally. Vectors can be independently controlled:

Canvas, WebGL, Audio, Navigator, Screen, Timezone, WebRTC, Fonts, Client Rects, Plugins, Battery, Connection

Domain Whitelist

Add domains that should never be containerized or fingerprint-spoofed. Useful for internal sites, local services, or sites that break with container isolation.

Container Management

Full table of all managed containers with per-container controls:

• Toggle spoofing on/off
• Regenerate fingerprint
• Delete container (removes all cookies and data for that site)

Requirements

• Firefox 100+ or LibreWolf
• Containers must be enabled (privacy.userContext.enabled = true in about:config)

Recommended about:config settings

For maximum WebRTC leak protection, set these in about:config:

Setting	Value	Purpose
media.peerconnection.ice.default_address_only	true	Only use default route for ICE
media.peerconnection.ice.no_host	true	Prevent host candidate gathering
media.peerconnection.ice.proxy_only_if_behind_proxy	true	Force proxy-only mode

LibreWolf may already have some of these set by default.

Testing

A built-in test page is included at test/fingerprint-test.html. To use it:

1. Load the extension via about:debugging
2. Add a hostname alias (e.g. 127.0.0.1 containsite-test.site in /etc/hosts) — localhost is excluded from containerization
3. Start a local server: python3 -m http.server 8888 --bind 0.0.0.0
4. Open http://containsite-test.site:8888/test/fingerprint-test.html in a regular (non-private) window
5. Open the same URL in a different container tab and compare composite hashes

File structure

manifest.json          MV2 extension manifest
background.js          Container management, navigation, HTTP header spoofing
inject.js              Fingerprint overrides (exportFunction-based, 18 vectors)
lib/
  prng.js              Mulberry32 seeded PRNG
  fingerprint-gen.js   Deterministic seed → device profile generator
popup/
  popup.html           Container list UI
  popup.css            Styles
  popup.js             Toggle, regenerate, prune, reset controls
options/
  options.html         Full options page (opens in tab)
  options.css          Styles
  options.js           Vector toggles, whitelist, container management
test/
  fingerprint-test.html  Comprehensive fingerprint verification page
icons/
  icon-48.png          Toolbar icon
  icon-96.png          Extension icon

Build

No build tools required. The extension is plain JavaScript with no dependencies.

To package as .xpi:

zip -r ContainSite.xpi manifest.json background.js inject.js lib/ popup/ options/ icons/icon-48.png icons/icon-96.png

License

GPL-3.0