Ready to go Firefox responsive design mode presets in JSON format.
This solution doesn't work on Firefox in 51 & above! for now you could change the pref "devtools.devices.url" to point at your own JSON file.
- Open about:config
- Search for devtools.responsiveUI.presets
- Replace the value of the key with presets.json file's content
The list provided using Media Queries for Standard Devices and some other articles.