Catchpoint by Huong Truong

Catchpoint is a free, open-source Firefox extension for intercepting, inspecting, and rewriting HTTP requests before they reach the server. Create unlimited rules to modify headers, redirect, mock responses, delay, or block requests — with a live capture mode for manually inspecting and editing requests.

What it provides
🔧 Modify headers — add, change, or remove request/response headers
🔀 Redirect requests — send matching requests to another URL
🎭 Mock responses — return a fixed response without contacting the server
⏱ Delay requests — simulate slow network conditions
🚫 Block requests — cancel matching requests
👀 Live capture — pause matching requests and manually inspect/edit headers
📋 Request logging — see requests affected by your rules in real time
🔢 Unlimited rules — rules can be enabled/disabled and reordered
📦 Import/export — save and share rules as plain JSON
🧩 Flexible matching — contains, glob, and regex