Official Mozilla Documentation
Firefox Developer Edition is a version of Firefox tailored for developers, featuring the latest Firefox features and experimental developer tools. The current Developer Edition includes these updates to the developer tools:
The Core Tools
Page Inspector
View and edit page content and layout. Visualise many aspects of the page including the box model, animations, and grid layouts.
Web Console
See messages logged by a web page and interact with the page using JavaScript.
JavaScript Debugger
Stop, step through, examine, and modify the JavaScript running in a page.
Network Monitor
See the network requests made when a page is loaded.
Performance Tools
Analyze your site's general responsiveness, JavaScript, and layout performance.
Responsive Design Mode
See how your website or app will look and behave on different devices and network types.