Scale Firefox and Chrome on HIDPI

Firefox:
about:config
layout.css.devPixelsPerPx --> change value from 1.5 to 3

Chrome
Open a terminal 

sudo gedit /usr/share/applications/google-chrome.desktop

Find this line:

Exec=/usr/bin/google-chrome-stable %U

Change n into 1.5 to 3:

Exec=/usr/bin/google-chrome-stable --force-device-scale-factor=n %U

Save/close, relaunch Chrome.