What Browser Is Tor Browser Based On?

The Tor Browser is built directly on top of Mozilla Firefox, specifically its Extended Support Release (ESR) version. While it retains the core rendering engine and interface of Firefox, the Tor Project heavily modifies the browser to eliminate tracking vectors, enhance privacy, and route all web traffic through the encrypted Tor network.

The Foundation: Mozilla Firefox ESR

Tor Browser uses Mozilla Firefox ESR as its base rather than standard consumer Firefox. The ESR version is designed for organizations that require stability over rapid feature rollouts. This provides the Tor Project with a stable, long-term foundation that receives critical security patches without introducing frequent structural changes that could break privacy modifications or introduce unexpected security vulnerabilities.

Why the Tor Project Chose Firefox

The Tor Project selected Firefox as the base for several strategic reasons:

• Open-Source Codebase: Firefox’s fully open-source nature allows developers to inspect, audit, and modify every part of the browser’s source code.
• Gecko Engine: The Gecko rendering engine provides strong standards compliance and performance while remaining independent of Chromium-based dominance.
• Extensive Customizability: Firefox allows deep architectural modifications necessary to disable native telemetry, geolocation services, and persistent identifiers.

Key Modifications Made to Firefox

Although Tor Browser looks and feels like Firefox, it includes numerous under-the-hood patches to protect user anonymity:

• Traffic Routing via the Onion Network: All network requests are routed through three encrypted relays (Guard, Middle, and Exit nodes) to conceal the user’s IP address and location.
• Anti-Fingerprinting Protections: Standard browser details—such as screen resolution, installed fonts, and hardware specifications—are standardized across all users to prevent websites from uniquely identifying a device.
• State Separation and Disk Protection: Browsing history, cache, cookies, and form data are never written to the local disk and are automatically cleared when the session ends or when the “New Identity” feature is triggered.
• Integrated Privacy Extensions: Tor Browser comes pre-configured with security-focused add-ons, including NoScript, to block potentially malicious scripts and restrict WebAssembly and canvas-based fingerprinting.

By combining Firefox’s robust web platform with rigorous privacy engineering, the Tor Browser provides a modern browsing experience while maintaining strict defense against surveillance and traffic analysis.