I suspect that the lack of 1FUI is also part of why high DPI support is lagging pretty badly on Linux. In some window managers/distros you can get something that sort of/mostly works for built-in stuff, but once you launch a third party application like Firefox it usually doesn't end well. I'm sure the Firefox devs would gladly support high DPI screens on Linux if there were a single standard way to do so. There isn't, though, and there probably won't be one anytime soon due to the fragmentation of the platform.