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:
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.

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.