and here we are, back to square one.

modules/plugin/base/src/nsPluginHostImpl.cpp

@@ -2951,14 +2951,11 @@ LoadXPCOMPlugin(nsIComponentManager* aComponentManager,
   // To figure out the filename of the plugin, we'll need to get the
   // plugin's CID, and then navigate through the XPCOM registry to
   // pull out the DLL name to which the CID is registered.
-  NS_NAMED_LITERAL_STRING(prefix, "software/mozilla/XPCOM/classID/");
-  NS_ConvertASCIItoUCS2 suffix(aCID);
-
-  const nsAReadableString& prefixRef = prefix;
-  const nsAReadableString& suffixRef = suffix;
-  nsAutoString path(prefixRef+suffixRef);
+  nsAutoString path;
+  path.AppendWithConversion("software/mozilla/XPCOM/classID/");
+  path.AppendWithConversion(aCID);
     // this contortion to append is a hack until I can figure out why the right |operator+| is not found
-    //  on some platforms
+    //  and/or why it doesn't know what a |nsPromiseConcatenation| is ... on some platforms
 
   nsRegistryKey cidKey;
   rv = aRegistry->GetKey(nsIRegistry::Common, path.GetUnicode(), &cidKey);

modules/plugin/nglsrc/nsPluginHostImpl.cpp

@@ -2951,14 +2951,11 @@ LoadXPCOMPlugin(nsIComponentManager* aComponentManager,
   // To figure out the filename of the plugin, we'll need to get the
   // plugin's CID, and then navigate through the XPCOM registry to
   // pull out the DLL name to which the CID is registered.
-  NS_NAMED_LITERAL_STRING(prefix, "software/mozilla/XPCOM/classID/");
-  NS_ConvertASCIItoUCS2 suffix(aCID);
-
-  const nsAReadableString& prefixRef = prefix;
-  const nsAReadableString& suffixRef = suffix;
-  nsAutoString path(prefixRef+suffixRef);
+  nsAutoString path;
+  path.AppendWithConversion("software/mozilla/XPCOM/classID/");
+  path.AppendWithConversion(aCID);
     // this contortion to append is a hack until I can figure out why the right |operator+| is not found
-    //  on some platforms
+    //  and/or why it doesn't know what a |nsPromiseConcatenation| is ... on some platforms
 
   nsRegistryKey cidKey;
   rv = aRegistry->GetKey(nsIRegistry::Common, path.GetUnicode(), &cidKey);