Debugger.html has three main regions in its user interface and the tool will be available in the Firefox Nightly version of Firefox DevTools.
1. Source Panel - Displaying current source for the application being debugged.
2. Editor Panel - It displays source files and settings breakpoints.
3. Right Sidebar - Consisting of call stack, current breakpoints, and variables when the debugger is paused.
Currently, Debugger.html is used in the debugging process of Firefox via Firefox Remote Debugging Protocol (RDP). Chrome browser and Node.js can also connect with debugger.html to debug chrome tabs and node processes respectively.
Mozilla is using React and Redux and as a result of it, they are shifting their Firefox tools away from XML User Interface Language (XUL). React and Redux simplifies code into smaller modules. Mozilla's Bryan Clark, Firefox developer tools product manager said, “We believe this will make the debugger and all of our future developer tools more approachable, predictable, understandable, and testable".
Read more on debugger.html here.