I am using Opensuse Tumbleweed on my Thinkpad T460s witha 14 inch
high-resolution screen (2560 x 1440 Pixel, WQHD).

To have a reasonable size for fonts and control elements, I added the line

ServerArguments=-nolisten tcp -dpi 144

in the [XDisplay] section of /etc/sddm.conf

This works perfectly in KDE and also with GTK applications under KDE such as
unison, and until yesterday, it also worked in Firefox.

But after running "zypper up" and restarting firefox (and also the whole system
later), the icons, menus and webpages in Firefox are too small. I have no idea
whether this new behavior is due to changes in Firefox or GTK (or elsewhere).
My Firefox version is:

> rpm -q MozillaFirefox
MozillaFirefox-51.0.1-1.2.x86_64

On

https://wiki.archlinux.org/index.php/HiDPI

I found the option to do "export GDK_SCALE=2" before starting Firefox. This
works, but makes Firefox too big for my purposes (and apparently only integer
scaling factors are allowed).