High DPI screen and i3 + Firefox

[Monq]

I'm trying to run i3 on a high DPI screen. By that I mean a screen with a slightly higher DPI than average (sorta 125 %, according to Windows), not a 4K screen.

To rescale Firefox, I'm supposed to use the GTK3 env variable GDK_SCALE. GDK_SCALE=2 makes things huge and, unfortunately, that env variable only works with integers.

[Monq]

I was under the impression that GTK3 applications simply ignored the Xft.dpi and xrandr --dpi options, but they don't! Only Firefox ignores them.

So in about:config I have layout.css.devPixelsPerPx = 1.2 and it works fine. The rest of GTK3 applications are well behaved and they scale as they should, so problem solved.