Timezone Guard от Three_radiance

Timezone Guard is a lightweight, privacy-focused Firefox extension that protects you from timezone-based and User-Agent fingerprinting.

What it does:
• Overrides your browser's timezone across all contexts — main page, iframes, Web Workers, and Service Workers
• Optionally spoofs your User-Agent (navigator.userAgent, platform, language, languages, userAgentData, maxTouchPoints, oscpu)
• Optionally spoofs outgoing HTTP headers (Accept-Language and User-Agent) to match your selected timezone and browser identity
• Keeps all spoofed values internally consistent so advanced trackers can't detect the mismatch
• Works alongside Firefox's built-in privacy.resistFingerprinting without conflicts
• Bypasses strict Content-Security-Policy (CSP) on sites like Spotify and YouTube
• Toggle the extension on/off or disable header spoofing independently from the popup

How it's different:
Unlike other extensions that only patch the main window, Timezone Guard handles nested iframes, Dedicated Workers, Shared Workers, and Service Workers — the places where trackers often hide. It also patches the full Date API (constructor, getters, setters, parse), Intl.NumberFormat, and Temporal API when available.

No data collection. No remote servers. Fully open source.

GitHub: https://github.com/3Radiance/Timezone-Guard