Tamper Data Classic by SamDev

Tamper Data lets you intercept, view, and modify HTTP/HTTPS requests in real time — directly from your browser.
Inspired by the classic Tamper Data extension, this modern rewrite brings back the ability to inspect and manipulate web traffic without relying on external proxy tools.
Features:

Monitor all outgoing HTTP and HTTPS requests in real time
View and edit request headers before they are sent
Modify POST data on the fly
Filter requests by URL, method, or content type
Pause, tamper, and release individual requests
Lightweight and privacy-friendly — no data is collected or transmitted

Use cases:

Web development and debugging
Security testing and penetration testing
API troubleshooting
Learning how web requests work under the hood

All processing happens locally in your browser. No data ever leaves your machine.