Sorry Link should be: https://developer.mozilla.org/en-US/docs/Tools/Responsive_Design_Mode

There is a devtools Settings button on the right side (gearwheel icon).

Press it and select "Responsive Design Mode".

Thank you TyDraniu – but I am able to access "Responsive Design Mode" OK, its presentation is where the problem is. Before I upgraded I was getting the newer presentation as described and shown on this page: https://developer.mozilla.org/en-US/docs/Tools/Responsive_Design_Mode – But since I updated Firefox to the latest version the presentation of "Responsive Design Mode" has reverted back to the old style – as shown on the attached screenshot – which only has limited presets and no specific device options.

You get the old appearance when multi-process is disabled. Only when multi-process is enabled you will get the new style that allows to select a device.

You can check the "Help -> Troubleshooting Information" page (about:support) to see why multi-process is disabled. You can also check these prefs on the about:config page to see if they have the default value.

• browser.tabs.remote.autostart
• browser.tabs.remote.autostart.2

You can open the about:config page via the location/address bar. You can accept the warning and click "I'll be careful" to continue.

• http://kb.mozillazine.org/about:config