To simplify site compatibility testing, it's useful to have a UA spoofing setting easily available in the Responsive design view screen. (It's a somewhat unfortunate consequence of the current architecture that this setting will probably change UA spoofing globally, not just for the window. If it's possible to fix that, great, if not we'll live with it.)

This seems like it is solved by the device list feature in new RDM shipping in Firefox 52.