Firefox seems not to read the GTK configuration, but it does will honor your ~/.fonts.conf which basically contains the font configuration, including the sub-pixel order.

My ~/.fonts.conf

<?xml version='1.0'?> <!DOCTYPE fontconfig SYSTEM 'fonts.dtd'> <fontconfig>

<match target="font">

<edit mode="assign" name="autohint">

<bool>false</bool>

</edit>

</match>

<match target="font">

<edit mode="assign" name="rgba">

<const>none</const>

</edit>

</match>

<match target="font">

<edit mode="assign" name="hinting">

<bool>true</bool>

</edit>

</match>

<match target="font">

<edit mode="assign" name="hintstyle">

<const>hintslight</const>

</edit>

</match>

<match target="font">

<edit mode="assign" name="antialias">

<bool>true</bool>

</edit>

</match> </fontconfig>

Link 1 (this)

Link 2 (about greyscale)

Link 3 (all available params for that xml)

---

IntelliJ IDEA configuration below