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:
View and edit page content and layout. Visualise many aspects of the page including the box model, animations, and grid layouts.
See the network requests made when a page is loaded.
Responsive Design Mode
See how your website or app will look and behave on different devices and network types.