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)
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IntelliJ IDEA configuration below
