The Responsive Design Mode available till now in Firefox browser has been redesigned and enabled by default in Firefox 52 Nightly. The new RDM looks awesome and even better when compared with Chrome’s, comes with redesigned UI, device selector and it has been completely rewritten using HTML, React and Redux and other technologies by Mozilla.
Firefox 52 devtools: New Responsive Design Mode
To see it in action, ensure e10s is enabled and you’re using latest version Nightly and press Ctrl+Shift+M to open or click on the Firefox menu, click Devtools and select Responsive Design Mode.
Want to see how the old RDM looks? Change this preference in about:config “devtools.responsive.html.enabled’ to false. Restart the browser. To get new RDM back, change the above preference value to true again.
Mozilla to add more features to RDM, including Network Throttling, Zoom to fit for working with large viewports and UI to change device pixelratio.