I'm using Ubuntu 16.04 and Firefox 50.1 with an external monitor attached to my laptop. My laptop screen is 14-inch 1080p which has a scale factor of 1.25 in System Settings. My external monitor is 22-inch 1080p which has a scale factor of 1. And in my about:config, I set the layout.css.devPixelsPerPx to -1.0.
When I set the scale factor to match the smallest controls in System Settings, Firefox becomes tiny on my laptop screen. Not completely unusable, but annoying.
Is there anyway to get per-monitor DPI scaling on Firefox? Especially natively without installing new extensions? Firefox can already handle this on Windows 10.