diff --git a/general/genlib/js91.xml b/general/genlib/js91.xml
index 282b41b555..9e3855efce 100644
--- a/general/genlib/js91.xml
+++ b/general/genlib/js91.xml
@@ -95,12 +95,15 @@
       <xref linkend="icu"/>,
       <xref linkend="rust"/>, and
       <xref linkend="which"/>
-      <!-- Should we list llvm here? -->
     </para>
 
     <bridgehead renderas="sect4">Recommended</bridgehead>
     <para role="recommended">
-      <xref linkend="llvm"/> (with <application>Clang</application>)
+      <!-- If clang is installed, it will be used instead of gcc.
+           gcc does not work for 32-bit system:
+           https://bugzilla.mozilla.org/show_bug.cgi?id=1729459 -->
+      <xref linkend="llvm"/> (with <application>Clang</application>,
+      required for 32-bit system)
     </para>
 
     <!-- It seems nasm is only used for aarch64-win64. -->