How to open the Responsive Design Mode?
There are two ways to use the Responsive Design Mode, first way is using the mode along with DevTools and second way is using the mode without DevTools.
With DevTools
Right-click in the web page and click Inspect from the context menu that appears. The DevTools will appear in the DevTools click the responsive design mode button present at the top-right corner of DevTools or just use the shortcut “Ctrl+Shift+M”.
Without DevTools
Click the application menu option which appears as three bars and it’s present in the top-right corner of the browser. Then navigate to More tools > Responsive Design Mode. Alternatively you can directly open the mode using the shortcut “Ctrl+Shift+M”.
Responsive Design Mode in Mozilla Firefox Browser
All web pages which are design for multiple devices having varying screen width must have a responsive design. These design can have relative layout and/or reflowing content. To inspect the responsive design of a web page Firefox provides a mode for simulating various screen sizes, throttling and touch.