React Quantum by Mozilla is a tool for building and debugging React applications. It's designed to aid developers in quickly locating and solving bugs, as well as help them create and maintain stateful React applications. With React Quantum, you can gain insights into the performance of your application and make informed decisions about whether changes you make will increase or decrease performance.

React Quantum enables developers to see and edit the component hierarchy of their React application directly on the page, allowing them to troubleshoot issues and perform changes without navigating away from the page. It also provides a useful visualization that shows the number of re-renders that have occurred over time, which helps developers pinpoint where the application is spending its time.

In addition, developers can add an optional custom heatmap to the component tree, which makes it easy to spot and diagnose performance issues on a page.