Fix font aliasing on flatpak GTK programs under Plasma. Create directory, for example ~/.var/app/io.gitlab.librewolf-community/config/fontconfig then put this file in it.

fonts.conf

<?xml version='1.0'?>
<!DOCTYPE fontconfig SYSTEM 'fonts.dtd'>
<fontconfig>
 <its:rules version="1.0" xmlns:its="http://www.w3.org/2005/11/its">
  <its:translateRule selector="/fontconfig/*[not(self::description)]" translate="no"/>
 </its:rules>
 <!-- 
 Artificial oblique for fonts without an italic or oblique version
 -->
 <match target="font">
  <!-- check to see if the font is roman -->
  <test name="slant">
   <const>roman</const>
  </test>
  <!-- check to see if the pattern requested non-roman -->
  <test name="slant" compare="not_eq" target="pattern">
   <const>roman</const>
  </test>
  <!-- multiply the matrix to slant the font -->
  <edit name="matrix" mode="assign">
   <times>
    <name>matrix</name>
    <matrix>
     <double>1</double>
     <double>0.2</double>
     <double>0</double>
     <double>1</double>
    </matrix>
   </times>
  </edit>
  <!-- pretend the font is oblique now -->
  <edit name="slant" mode="assign">
   <const>oblique</const>
  </edit>
  <!-- and disable embedded bitmaps for artificial oblique -->
  <edit name="embeddedbitmap" mode="assign">
   <bool>false</bool>
  </edit>
 </match>
 <!--
 Synthetic emboldening for fonts that do not have bold face available
 -->
 <match target="font">
  <!-- check to see if the weight in the font is less than medium which possibly need emboldening -->
  <test name="weight" compare="less_eq">
   <const>medium</const>
  </test>
  <!-- check to see if the pattern requests bold -->
  <test name="weight" compare="more_eq" target="pattern">
   <const>bold</const>
  </test>
  <!--
		  set the embolden flag
		  needed for applications using cairo, e.g. gucharmap, gedit, ...
		-->
  <edit name="embolden" mode="assign">
   <bool>true</bool>
  </edit>
  <!--
		 set weight to bold
		 needed for applications using Xft directly, e.g. Firefox, ...
		-->
  <edit name="weight" mode="assign">
   <const>bold</const>
  </edit>
 </match>
 <match target="font">
  <edit name="rgba" mode="assign">
   <const>none</const>
  </edit>
 </match>
 <match target="font">
  <edit name="hinting" mode="assign">
   <bool>true</bool>
  </edit>
 </match>
 <match target="font">
  <edit name="hintstyle" mode="assign">
   <const>hintslight</const>
  </edit>
 </match>
 <match target="font">
  <edit name="antialias" mode="assign">
   <bool>true</bool>
  </edit>
 </match>
 <!-- Disable bitmap fonts. -->
 <selectfont><rejectfont><pattern>
  <patelt name="scalable"><bool>false</bool></patelt>
 </pattern></rejectfont></selectfont>
</fontconfig>