Tracking down some impurities, I found that the following snippet from ~/.config/fontconfig/fonts.conf is another factor required for the issue to occur:

 <match target="font">
  <edit mode="assign" name="antialias">
   <bool>true</bool>
  </edit>
 </match>

It was presumably put there by KDE Plasma like the rest of the file, and I have no idea why the heck it breaks on Firefox 136.

Is there any way to disable this patch in buildMozillaMach since i am working on buildMozillaMach for zen browser at https://github.com/Eveeifyeve/nixpkgs/tree/zen-browser-moz?

Running phase: unpackPhase
@nix { "action": "setPhase", "phase": "unpackPhase" }
unpacking source archive /nix/store/fbck76hxp37rdsaiqqjdaq7g2yp97imc-source
source root is source
Running phase: patchPhase
@nix { "action": "setPhase", "phase": "patchPhase" }
applying patch /nix/store/vxk7kwxf60bppmblksx8j70w3j7ggz4h-env_var_for_system_dir-ff133.patch
can't find file to patch at input line 5
Perhaps you used the wrong -p or --strip option?
The text leading up to this was:
--------------------------
|diff --git a/toolkit/xre/nsXREDirProvider.cpp b/toolkit/xre/nsXREDirProvider.cpp
|index 6db876975187..5882c5d7f1d6 100644
|--- a/toolkit/xre/nsXREDirProvider.cpp
|+++ b/toolkit/xre/nsXREDirProvider.cpp
--------------------------
File to patch:
Skip this patch? [y]
Skipping patch.
2 out of 2 hunks ignored