DPI value doesn't work well with multiple screens

[x-ji]

Say I have a MacBookPro with 192 dpi and an external monitor with 96 dpi, then the unified dpi will force only one display size of Firefox, and it will be broken on one screen or another. I had to manually reset layout.css.devPixelsPerPx in about:config to fix the issue. It should allow for multiple dpi values on different screens.

[RealSkeptic]

I join this. Actually, I think it should be marked as a bug rather than an enhancement. When I upgraded to my iMac retina, Firefox worked well (I had all the settings from my old Mac). My secondary screen is 4k, my main screen is 5k (retina). And OS X somehow does an amazing job of keeping the windows, graphics and fonts the same size when you move the window from one screen to the other and even put it in the middle.

But I was trying to play around with Tabkit preferences today, and saw the DPI setting, so I changed it. And from that moment on, Firefox went crazy. If I set it to 96, the window on the main screen would be tiny, and have wrong layout. if I set it to 192, the fonts on the windows on the secondary screen went huge. The setting should not behave like this. Perhaps it should even be deleted or disabled when more than one screen is detected, it would be less dangerous (I eventually had to restore my user profile from Time Machine to overcome this).